## Wednesday, 12 November 2014

### Reference Notes for Science Class X

Force

1)    In which direction does the force of gravity act?                                                                        Ans: The direction of force of gravity is always acting towards the centre of heavenly body.

2)    Define null point.
Ans: The space between the gravitational fields of two heavenly bodies, at which the resultant gravity is zero, is called null point.

3)    A satellite does not need any energy to revolve around the earth, why?
Ans:  While a satellite is revolving around the earth, the centripetal force due to gravity of the earth is equal to the centrifugal force due the motion of the satellite. So, a satellite does not need any energy to revolve around the earth.

4)    In which condition do feather and coin dropped from the same height fall at the same speed? What is the acceleration of the feather and the coin at that instant? Justify your answer.
Ans:  A feather and a coin dropped from the same height falls at the same speed at the condition of free fall. At that condition of free fall, both coin and feather gain same acceleration of 9.8m/s2. Because the acceleration produced in the freely falling bodies, whatever their masses, are same in the absence of air resistance.

5)    Write one effect seen on the ocean or sea due to the gravitation of the moon and the sun.
Ans:  Tides in sea and ocean are due to the gravitation of the moon and the sun.

6)    Where does the weight of the body become maximum in Nepal?
Ans:  The weight of the body become maximum at the Terai region of Nepal because as we know that, the weight of the body depends upon the acceleration due to gravity (g) and the value of 'g' is inversely proportional to square of the radius of the earth (i.e. g  1/R2 ). The radius of the earth at the Terai region is less than that of mountain region as the earth is not a perfect square.

7)    Why does a sheet of paper fall later on the earth surface than that the paper ball of equal weight if thrown from a certain height?
Ans:   A sheet of paper fall later on the earth surface than that the paper ball of equal weight if thrown from a certain height because the sheet of the paper will experience a larger air resistance due to its larger surface area but the paper ball has less surface area so that air interferes less on paper ball than that of paper sheet.

8)    Where does the weight of the body become maximum on the surface of the earth?
Ans:  As we know that, the weight of the body depends upon the acceleration due to gravity (g) and the value of 'g' is inversely proportional to square of the radius of the earth (i.e. g  1/R2). The polar radius of the earth is less than the equatorial radius as the earth is not a perfect square. Hence, the weight of the body becomes maximum at the pole on the surface of the earth.

9)    What are the two factors that affect the force of gravitation?
Ans:  The two factors that affect the force of gravitation are:-
a)    Product of masses of any two bodies and
b)    Square of the distance between their centres

10)  The weight of the object is more at polar region than equatorial region, why?
Ans:  As we know that, the weight of the body depends upon the acceleration due to gravity (g) and the value of 'g' is inversely proportional to square of the radius of the earth (i.e. g  1/R2). The polar radius of the earth is less than the equatorial radius as the earth is not a perfect square. Hence, the weight of the body is more at the polar region than at the equatorial region.

11)  When a person jumps from the aeroplane with parachute can land safely on the earth surface but it is not possible on the moon, why?
Ans: When a person jumps from the aeroplane with parachute can land safely on the earth surface because on the earth there are layers of different gases. When a parachutists jump out of the aeroplane, parachute opens and expands due to which air exert large resistance on the parachute so that parachute fall down slowly and uniformly. But it is not possible on the moon because there is the absence of atmosphere on the moon. So, upthrust of air is not acted on parachute and falls with acceleration equal to acceleration due to gravity and strict with more force on the surface of moon.
12)  Write two differences between gravity and gravitation.
 S.N Gravitation S.N Gravity a) It is the mutual force of attraction between any two object in the universe. a) It is the force of attraction of the heavenly body to any object towards its centre. b) It does not measure the weight of body. b) It measures the weight of the body. c) It is universal force. c) It is not universal.

13)  What is free fall and weightlessness?
Ans:  When a body is falling freely towards the centre of the earth under the influence of the gravitational pull of the earth without any external resistance, the fall of the body is called free-fall.
Weightlessness of a body is the state in which the body feels that it is not being attracted by any force.

14)  Why satellites not fall while revolving around Earth?                                                                                                      Ans: satellites not fall while revolving around sun due to the balance of two factors:                                           a) The gravitational pull between the Earth and the satellite i.e. centripetal force.
b) Velocity or the speed at which it would travel in a straight line i.e. centrifugal force.

15)  What effect will be on acceleration due to gravity if radius of the earth is decreased?
Ans: The magnitude of the acceleration due to gravity increase if radius of the earth is decreased by keeping its mass constant.

16)  What is gravitational constant? Write its S.I. unit and value?
Ans:  Universal gravitational constant 'G' can be defined as the force of gravitation that is exerted between two unit masses which are separated from one another by a unit distance.
Its S.I. unit is Nm2/kg2 and its value is 6.67 10-11 Nm2/kg2.

17)  At what condition the value of acceleration due to gravity is approximately zero, while getting down from parachute?
Ans:  When a parachute falls down, it is pulled downward by a certain force of gravity. Air also provides resistance and opposes the downward movement of parachute. When air resistance is equal to the pulling force of gravity of the earth, the acceleration of the falling parachute is approximately zero, while getting down from parachute.

18)  What is gravity?
Ans:  The force of attraction of the heavenly body to any object towards its Centre is called gravity.

19)  We do not get hurt, when we jump with parachute?
Ans:  On the earth there are layers of different gases. When a parachutists jump out of the aeroplane, parachute opens and expands due to which air exert large resistance on the parachute so that parachute fall down slowly and uniformly. Hence, the parachutists do not get hurt, when they jump with parachute out of an aeroplane.

20)  Define acceleration due to gravity (g). Write the S.I. unit of weight of an object?
Ans: The uniform acceleration produced in a freely falling body due to the gravitational pull of the earth towards its Centre is called acceleration due to gravity.
The S.I. unit of weight of an object is Newton (N).

21)  State Newton's Universal Law of Gravitation.
Ans: It states that, "everybody in the universe attracts every other body with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres."

22)  A piece of stone and a feather are dropped from the same height at lunar surface, which one will reach on the surface fast and why?
Ans:  As we know that there is no atmosphere in the lunar surface. When a piece of stone and a feather are dropped from the same height at lunar surface, they fall simultaneously due to free fall. Because acceleration produces in the freely falling bodies, whatever their masses, are the same in the absence of air resistance.

23)  The probability of getting hurt is more when a man jumps from greatest height, why?
Ans:  As we know that force is directly proportional to the acceleration due to gravity (i.e. F  g). If a person jumps from a certain height, his acceleration goes on increasing towards the earth surface. More the acceleration, with more force he strict on the earth surface. Hence, the probability of getting hurt is more when a man jumps from greatest height.

24)  What is gravitation?
Ans:  The mutual force of attraction between any two bodies in this universe is called gravitation.

25)  What is the difference between fall of parachute on the earth and that of the moon?
Ans: Difference between fall of parachute on the earth and that of the moon are:
 S.N. Fall of parachute on the earth S.N. Fall of parachute on the moon a) Parachute acquires upthrust of air so fall is with uniform velocity. a) Upthrust of air is not acted on parachute and falls with acceleration equal to acceleration due to gravity. b) Freefall is not experienced. b) Freefall and weightlessness is experienced.

26)  What is the effect of gravity on the falling object?
Ans: When object is falling, gravity provides necessary acceleration on it which is always acting towards the centre of the heavenly body.

27)  What is the value of acceleration due to gravity at equatorial region of the earth? Write in S.I. unit?
Ans:  The value of acceleration due to gravity at
a)    Equatorial region:- 9.78 m/s2
b)    Polar region:- 9.83 m/s2

28)  The acceleration due to gravity in the moon is 1.67m/s2. What does it mean?
Ans: It means that when the object falls towards the surface of the moon under the action of gravity, the velocity of the falling object increases at the constant rate of 1.67m/s in each second during its fall if the air resistance is neglected.

29)  What is gravitational field and gravitational field intensity?
Ans: The space around a mass over which it can exert gravitational force on other masses is called the gravitational field of the mass.
The gravitational field intensity at a point in the gravitational field is defined as the force experienced by unit mass placed at that point.

30)  A feather and a coin fall simultaneously on the surface of moon, why?
Ans:  As we know that on the surface of the moon, there is no air resistance (i.e. no atmosphere). If there is no air resistance, the acceleration produces in the freely falling bodies, whatever their masses, are the same. Therefore, if a feather and a coin are dropped simultaneously on the surface of moon, their speed increases with the same rate. As a result they reach the ground together.

31)  It is difficult to lift a large stone on the surface of the earth but easy to lift small one, why?
Ans:  To lift a load, we have to apply the force equal to the weight of the load. The load of an object depends on the force of gravity and its mass. The force of gravity is more in the large stone as it has more mass then in small stone. Due to this, large stone has more load than small stone. As a result, it is difficult to lift a large stone on the surface of the earth but easy to lift small one.

32)  Stone of 1kg and 5kg masses are dropped from the same height at lunar surface, which one will reach on the surface fast and why?
Ans:  When two stone of 1kg and 5kg masses are dropped from the same height at lunar surface, both will reach on the surface simultaneously. It is because at the lunar surface, there is no air resistance (i.e. no atmosphere). If there is no air resistance, the acceleration produces in the freely falling bodies, whatever their masses, are the same.

33)  Differences between mass and weight.
 S.N Mass S.N Weight a) Mass is the total amount of matter contained in a body. a) Weight of an object is the force with which the object is attracted towards the Centre of the heavenly body or the earth. b) It is scalar quantity. b) It is vector quantity. c) It is measured in kilogram (kg). c) It is measured in Newton (N).

34)  What is the effect of earth's gravity?
Ans: All the bodies present inside the gravitational field of a planet fall towards the surface of the planet due to the effect of gravity of that planet. Gravity produces an acceleration on the falling bodies.

Pressure

1.   “Only the object having the density less than water floats.” Is the statement true?  Why or why not? Give reasons.

Ans:  “Only the object having the density less than water floats.” This statement is false because object having more density can also float in water if it is provided with appropriate shape. For Instant: Iron has more density than water but the sheep made from iron does not sink and float easily in water.

2.   What is difference between the immersion of hydrometer in pure water and salty water?
Ans:  When hydrometer is immersed in pure water and salty water, more portion of hydrometer immerses in pure water whereas less portion of hydrometer immerses in salty water because the density of salty water is greater than pure water as a result it experience more upthrust.
3.   State law of floatation and name the instrument based on it.
Ans:  It state that, "For a body to float, the weight of the floating object should be equal to the weight of the liquid by it."
Hydrometer is the instrument based on it.
4.   Define pressure.
Ans:  The force acting normally per unit area of the surface is called pressure. Its S.I. unit is N/m2 or Pascal (Pa).
5.   Prove the relation of pressure exerted by an object with its surface area the applied force.
Ans:   Let us suppose, force 'F' is applied normally to an area 'A' and pressure 'P' is produced which is shown in the figure.
We know that, By the definition of pressure
Pressure is directly proportional to force applied.
i.e. P Î± F................. (i)
Pressure is inversely proportional to the area on which force acts.
i.e. P Î± 1/A  .............. (ii)
Combining (i) and (ii), we get
Î± F/A
Or, P = kF/A              (where k is constant)
If 1 N force is applied in 1 m2 then pressure will be 1 N/m2 and thus k = 1
Or, p = 1*F/A
...    p = F/A    proved
6.   By which two properties of liquid is it used in hydraulic break?
Ans: The two properties of liquid that is used in hydraulic break are as follow:-
(i)    Liquid transmit pressure equally in all the directions.
(ii)   Liquid cannot be compressed.
7.   What is hydrometer?
Ans: A device which is used to measure the specific gravity and density of liquids is called hydrometer.
8.   Name the device which works on the basis of Pascal’s Law.
Ans: The device which works on the basis of Pascal’s Law are:
a)    Hydraulic Brakes
b)    Hydraulic Press
c)    Hydraulic Garage Lift
9.    "The weight of liquid displaced is equal to the weight of floating object."  Which principle does this statement belong to?
Ans: The above statement belongs to 'Law of Floatation'.
10.   What difference will you get from the flying of air filled balloon and hydrogen filled balloon?
Ans: The density of hydrogen is less than that of the air. So, the balloon filled with hydrogen rises up faster than the balloon filled with air.
11.   One feels difficult to breathe in the deep well while cleaning it, why?
Ans: Carbondioxiode is the densest gas. It is heavier than air. In the well, Carbondioxiode displaces the air present inside the well and only Carbondioxiode gas is collected at the bottom. So, there is lack of oxygen. Hence the person feels difficult to breathe while cleaning the deep well.
12.  Write any two differences between density and relative density.
Ans: Differences between density and relative density are:
 S.N Density S.N Relative density a) It is the mass per volume of the substance. a) It is the ratio of the density of the substance to the density of water at 40C. b) Its S.I. unit is Kg/m3. b) It has no unit.

13.  Why a stone does sinks in water?
Ans: The density of stone is more than that of water. So, the stone cannot displace the water equal to its own weight. As a result, a stone sinks in water.
14.  What is the relation between the density of liquid and the upthrust exerted by the liquid?
Ans: Upthrust exerted by the liquid is directly proportional to the density of liquid.
i.e. U = vdg
Or, U Î±  d
Hence, upthrust increase with increase in density and vice versa.
15.  Write the relation between upthrust and volume of displaced liquid.
Ans:  Upthrust is directly proportional to the volume of displaced liquid.
i.e. U = vdg
Or, U Î±  v
Hence, upthrust increase with increase in volume and vice versa.
16.  Lifting a stone in water is felt to be comparatively lighter, why?
Ans:  Lifting a stone in water is felt to be comparatively lighter because when a stone immersed in water, the water exerts an upward force called upthrust on a stone. Due to this upthrust the downward force on a stone reduced. Thus, apparent weight of the stone is far less than its real weight.
17.  What is hydraulic press?
Ans:  A hydraulic press is a simple machine which convert small force into large force and vice versa.
18.  Prove: P = hdg
Solution:
Let,    p = pressure of liquid
d = density of liquid
h = depth of liquid column
g = acceleration due to gravity
And A = cross sectional area of container
We know that, By the definition of pressure
p =   F/A
or, p =     mg/A                       (F = mg)
or, p =     d*v*g/A                 (m = d×v)
or, p =       d*A*h*g/A          (v = A×h)
p =   hdg    proved

19.  What is relative density?
Ans: The ratio of the density of the substance to the density of water at 40C is called relative density.
20.  What number will be shown by hydrometer if sunk in pure water?
Ans: 1000kg/m3 number will be shown by hydrometer if sunk in pure water.
21.  On what factors liquid pressure depends?
Ans: The factor on which liquid pressure depends is:
a)    The density (d) of liquid
b)    The depth (h) of liquid column
c)    The value of acceleration due to gravity (g).
22.  When a ball is pressed in water, why does it try to come up?
Ans: When a ball is pressed in water, the water exerts an upward force called upthrust on it. Due to this upthrust the downward force on the ball is reduced. Therefore, a ball tries to come up when it is pressed in water.

23.  If a hydrometer is immersed in sugar solution and in pure water, in which it immersed more, why?
Ans:  The hydrometer immersed more in pure water than in sugar solution because as we know that upthrust is directly proportional to the density of the liquid (i.e. U d). The density of sugar solution is more than that of pure water.
24.  What is density?
Ans:  The mass per volume of the substance is called density.
25.  Write the formula based on Pascal’s Law.
Ans:  The formula based on Pascal’s Law is F1 / A1 = F2 / A2
26.  An iron ball sinks in water but ship made up of iron floats on water, why?
Ans:  If the weight of the water displaced by a sinking body is greater than the weight of the body, the body floats on water. Otherwise, it sinks. In case of iron piece, the weight of the displaced liquid by it is less than its weight. Hence, it sinks in water. But in the case of ship made of iron, it is designed in such a way that the weight of liquid displaced by it is greater than its weight. Hence it floats on water. Therefore, an iron ball sinks in water but ship made up of iron floats on water.
27.  When a bucket full of water is pulled up from a well, in which condition either inside or outside the water it is difficult to lift up and why?
Ans:  When a bucket full of water is pulled up from a well, it become difficult to lift up when the bucket is outside the water because the upthrust provided by air is very less than the upthrust provided by water in the well.
28.  Write any two differences between force and pressure.
Ans:  Differences between force and pressure are:
 S.N. Force S.N. Pressure a) Its S.I. unit is Newton (N). a) Its S.I. unit is Pascal or Nm­-2. b) It is vector quantity. b) It is scalar quantity.

29.  State Archimedes principle.
Ans:  It states that, "When a body is partially or wholly immersed in a liquid, it experiences an upthrust which is equal to the weight of the liquid displaced by it."
30.  At what condition a body floats on liquid?
Ans:  A body floats on a liquid when the weight of liquid displaced by it is equal to its own weight.
31.  It is easier to swim on the sea than on the pond, why?
Ans:  As we know that upthrust is directly proportional to the density of the liquid (i.e. U d). The density of sea water is more than that of pond water. Due to this, when a man swims, the upthrust acting on him due to sea water  is greater than the upthrust acting on him due to the pond water. Because of this reason, it is easier to swim on the sea than on the pond.
32.  State Pascal's law.
Ans:  It states that, "When pressure is applied on a liquid enclosed in a vessel, it is transmitted equally in all the directions."
33.  Weight of an object is decrease when immersed in water, which law does this statement represent? How much weight will an object lose in water?
Ans:  Weight of an object is decrease when immersed in water. The above statement belongs to Archimedes' Principle. This is because of upthrust given by water to that object.
The weight lost by an object in water is equal to the upthrust given by water or the weight of the water displaced by that object.
34.  Lower portion of dam is made stronger than upper portion, why?
Ans:  As we know that, pressure is directly proportional to the depth of liquid (i.e. p Î± h). If the bases of dame are made wide, the pressure exerted by the dam on the ground will be small. As a result, the dam is avoided from descending into the ground. In addition to this, the dame has to withstand high pressure due to the stored water in the pond. For these reason, lower portion of dam is made stronger than upper portion.
35.  Prove: p Î± h.
Solution:
Let,    p = pressure of liquid
d = density of liquid
h = depth of liquid column
g = acceleration due to gravity
And A = cross sectional area of container
We know that, by the definition of pressure
p =   F/A
or, p =     mg/A                       (F = mg)
or, p =     d*v*g/A                 (m = d×v)
or, p =       d*A*h*g/A          (v = A×h)
p =   hdg

For a given liquid at a given place density of the liquid and acceleration due to gravity are constant.
... P Î± h
36.  What is the role of water in hydraulic press to multiply small force into large one?
Ans:  Water plays a great role to transmit the pressure equally, exerted by the small piston, perpendicularly to all sides of the closed container. As a result, force is multiplied in the larger piston.
37.  Define one Pascal pressure.
Ans:  If 1N force is applied in 1m2 area then the pressure is said to be one Pascal.
38.  Mention the use of hydrometer.
Ans:  The uses of hydrometer are:
a)    It is used to measure the specific gravity and density of liquids.
b)    Lactometer is a type of hydrometer used to measure the purity of milk.
39.  Why does hydrometer have heavy bulb and narrow stem?
Ans: Hydrometer has heavy bulb and narrow stem because heavy bulb helps hydrometer to float in a liquid vertically and narrow stem provide greatest sensitive on reading.
40.  An egg sinks in pure water but floats on salty water, why?
Ans:  As we know that, upthrust is directly proportional to the density of the liquid (i.e. U d). The density of salty water is more than that of pure water. So, the weight of the pure water displaced by the egg is less than the weight of the egg. Hence, the egg sinks in pure water. But the weight of the salt water displaced by the egg is greater than the weight of the egg. Hence, the egg floats in salty water. Therefore, an egg sinks in pure water but floats on salty water.
41.  Why it is easier to pull the bucket of water from the well until it is inside the water but difficult out of water?
Ans:  When a bucket remains inside the water, it experiences an upthrust due to water. So, it is easier to pull the bucket of water from the well until it is inside the water. But it difficult to lift up when the bucket is outside the water because the upthrust provided by air is very less than the upthrust provided by water in the well.
42.  The weight of any object decreases inside water. Give reason.
Ans:  When an object immersed in water, the water exert an upward force called upthrust on the body. Due to this upthrust the downward force on the body reduced. Therefore, the weights of any object decreases inside water.
43.  A ship coming from the sea enters the river, will it hull sinks more or less in river water? Give reason.
Ans:  The hull of the ship sinks more in river water. As we know that, upthrust is directly proportional to the density of the liquid (i.e. U d). The density of river water is less than that of sea water. So, the upthrust provided by the river water is less than the upthrust provided by the sea water.
44.  A hole at the bottom of the boat is more dangerous than that at the side, why?
Ans:  The bottom of the boat exert more pressure, as pressure is directly proportional to the depth of liquid (i.e. p h), than that at the sides. If there is hole at the bottom of the boat then liquid enters with large pressure inside the ship but at the top it enters slowly. Therefore, a hole at the bottom of the boat is more dangerous than that at the side.
45.  Name the device and their uses which work on the basis of Pascal’s Law.
Ans: The device which works on the basis of Pascal’s Law are:
(a)   Hydraulic Brakes
A hydraulic break is a breaking system which is used in stopping four wheelers and other heavy automobiles.
Uses:
(1)     It is used to stop heavy automobiles like trucks, buses, aeroplanes etc. by applying a small force.
(b)   Hydraulic Press
A hydraulic press is a simple machine which convert small force into large force and vice versa.
Uses:
(1)   It is used for compressing cotton goods, paper, etc. in industries.
(2)   It is used for extracting oil from oil seeds.
(c)   Hydraulic Garage Lift
Hydraulic Garage Lift is a machine used to lift light vehicles in service stations during their services,
Uses:
(1)   It is used for lifting automobiles like car, bike, bus, etc. in service station.
46.  If an object immersed in water at what condition it can sinks on water and at what condition it can floats on water?
Ans:   If the density of an object is greater than the density of the liquid, the body completely sinks in the liquid and lies on the bottom of the container.
If the density of an object is equal to the density of the liquid, the body completely immersed but remains floating in the body of the liquid.
If the density of an object is less than the density of the liquid, only a part of the body remains inside the liquid. The portion of the body which lies inside the liquid displaces the liquid equal to the weight of the body.

47.  The ice made of water floats on water, why?
Ans:  The sinking and floating of a substance depends on density of a substance and density of a liquid. If the density of substance is less than the density of liquid then substance floats on liquid. The density of ice is slightly less than the density of water. Therefore the ice made of water floats on water.
48.  The different weight of a piece of stone weighing in three different media air, water and solution of common salt with water are given in the table below. Answer the following question.
 Medium A B C Weight 15N 20N     17N

a)    Which should be water and which one the solution of common salt with water out of three medium A, B and C.
Ans: Medium C should be water and medium A should be the solution of common salt with water out of three medium A, B and C.
b)    If the weight of 1Kg of mass in air is 10N, find out the mass of the piece of stone?
Ans: Here given,
Weight of stone in air (Wa) = 20N
Stone of weight 10N has mass = 10kg
Stone of weight 1N has mass = (1/10) kg
Stone of weight 20N has mass = (1/10) 20 = 2kg
c)    Find out the mass of water displace by the piece of stone?
Ans: Here given,
Weight of stone in air (Wa) = 20N
Weight of stone in water (Ww) = 17N
Mass of water displaced by stone =?
Now,
Weight of water displaced by stone = Wa - Ww
= 20-17
= 3N
...    Mass of water displaced by stone =    weight of displaced water/acceleration due to gravity
= 3/10
= 0.3 kg = 300gm
49.  If two water tanks, one (A) having larger base area than the other (B),
carry equal volume of water, explain whose bottom experience more pressure.

Ans: In the given diagrams, both tanks 'A' and 'B' contain equal volume of water. The height of liquid column in tank 'B' is more than       that of tank 'A' because of less cross-sectional area. Since the liquid pressure is directly proportional to the height of liquid column, the base of tank 'B' experiences more pressure than the base of tank 'A'.
50.   In the given figure, on which surface more pressure is exerted and why?

Ans: In the given diagram, both surfaces A and B experience same pressure because the height
of the liquid is equal in both container.
51.  The speed of flow of water out of a tap of upper floor is less than that of the down floor. Why?
Ans: We know that the liquid pressure is directly proportional to the height of the liquid column. The height of the liquid column on the down floor is more than that on the upper floor. So, the speed of flow of water out of a tap of upper floor is less due to less pressure and that in the lower floor is more due to more pressure of water.
52.  The gravity bulb of hydrometer is made heavier.
Ans: The gravity bulb of hydrometer is made heavier so that its centre of gravity remains very low, when it is floating in denser liquids.          It helps the hydrometer to float upright in stable equilibrium.
53.  A hydrometer is once put into fresh water and then it is put into salt water. Which one is fresh
water in between 'A' and 'B' ? Justify your answer.
Ans: Out of two container A and B, container A contains fresh water because more
portion of hydrometer has immersed in the water of the container A. As we know that, upthrust
is directly proportional to density of liquid and fresh water has less density than that of salt water.
So, it will experienced less upthrust in fresh water.
54.  The size of air bubble becomes bigger as it rises up in the water, why?
Ans: We know that the liquid pressure is directly proportional to the depth of the liquid from the free surface. The liquid pressure is more at the bottom of the water than its surface. Due to the more pressure at the bottom of the water, the size of air bubbles is small than that in the surface of the water. So, the size of air bubble becomes bigger as it rises up in the water due to decrease in water pressure.

Energy

1.  Write the name of two radioactive elements.

Ans: The name of two radioactive elements is:                                                                                                                                                   a) Uranium                                                                                                                                                                                                                  b) Plutonium                                                                                                                                                                                                           c) Radium

2.   Government of Nepal encourages the farmer to establish bio gas plan. Write any two scientific reasons for it.                                                                                                                                                                    Ans: Government of Nepal encourages the farmer to establish bio gas plan because of following reasons:                                                                    a) Biogas burns without smoke and hence does not cause air pollution.                                                                                                                                                           b) The by-product of biogas is good manure which increases the fertility of soil.                                                                                                                                              c) Biogas is cheap and renewable source of energy and can be generated easily.
3.   What amount of average solar energy does the earth surface receive per square meter?                                                     Ans: Amount of solar energy received by the earth surface per square meter per second is estimated to be 1.4 kW.
4.   Write any natural phenomena from which geothermal energy can be obtained.                                 Ans: Natural phenomena from which geothermal energy can be obtained are:                                   a) Volcano eruption                                                                                            b) Earthquake

5.   Write a major disadvantage of fossil fuel.                                                                                                         Ans: Disadvantages of fossil fuel are:                                                                                                          a) Fossil fuel causes air pollution when it is burnt.                                                                                                         b) When petrol is brunt, harmful substances like unbrunt hydrocarbons, carbon monoxide, oxides of nitrogen, sulphur are produced. These cause acid rain and thus soil and water pollution give innumerable problem to health.
6.   In fact the hydroelectricity is an indirect source of solar energy. Clarify the statement.                                                                                                  Ans: The heat from the sun evaporates the water from the surface of earth, from the tree leaves, ponds, rivers, oceans etc. the water vapour thus formed rises high in the atmosphere and becomes cloud. When it rains, the water vapour comes back on the earth in the form of rain, snow, hail etc. the rain water and the water formed by melting of the snow, hail, ice, etc. flow rapidly in the river and lakes which are used to produce hydroelectricity.
7.What is bio                                                   fuel?                                                                                                            Ans: Fuel such as methane produced from renewable biological resources such as plant biomass, animal dung, firewood, etc. is called bio fuel.
8.   It is difficult to produce hydroelectricity in Nepal? Give any two reasons.                                                                                                     Ans: It is difficult to produce hydroelectricity in Nepal because                                                                                                       a) Lack of development of the modern technology and equipment.                                                                                                     b) Nepal is a poor country and there is lack of means, resources and skilled human resource.
9.   What is the source of solar energy?                                                                                                      Ans: The main source of solar energy is the thermonuclear fusion reaction that continuously takes place in the sun.
10.  Energy contained in fossil fuels is the outcome of solar energy, why?                                                                                                       Ans: Fossil fuels are obtained from remains of animals and plants under the earth over a period of millions of years. Plants get energy from sun during photosynthesis and animal get solar energy indirectly from the plants. Therefore, Energy contained in fossil fuels is the outcome of solar energy.
11.  What is primary source of energy?                                                                                                                                                                                            Ans: The natural source of energy which can be used in the natural form is called primary source of energy.
12.  Write any two ways to reduce energy crisis.                                                                                                     Ans: Ways to reduce energy crisis are:                                                                          a) By using renewable sources of energy in place of non-renewable source of energy.                                                                                                       b) BY developing solar devices in place of petroleum devices.                                                                                                      c) By developing and using alternative energy sources.
d)  By avoiding unnecessary use of power consuming devices.
e)  By using existing sources of energy wisely and economically.
13.What.is.renewable.energy?                                                                                                    Ans: The energy which can be reproduced in short time and can be used continuously for long period of time is called renewable energy.                                                                                                                                                                                                                              For e.g.: Hydropower, Solar energy, Geothermal power, Wind energy, Tidal energy, Biomass energy etc.
14.  Write the differences between Deuterium and tritium.
 S.N Deuterium S.N Tritium a) It is the isotopes of hydrogen having atomic mass two. a) It is the isotopes of hydrogen having atomic mass three. b) It has one proton and one neutron. b) It has one proton and two neutrons. c) It is denoted by 1H2. c) It is denoted by 1H3.

15.  Write any two reasons that urbanization brings energy crisis.                                                                                         Ans: The main reasons for energy crisis due to urbanization are:                                                                                                                      a) Along with the increasing urbanization, the demand of petroleum produced has been increasing day by day to run automobiles, industries, etc. such enormous demand of energy brings energy crisis.                                                                                       b)  There is misuse of non-renewable source of energy due to urbanization.
16.What is energy crisis?                                                                                                                                 Ans: The situation in which there is acute scarcity of energy sources is called energy crisis.
17.  'The use of solar energy should be increased in context of Nepal', give any two reasons to justify the statements.                                                                                                                 Ans: The use of solar energy should be increased in context of Nepal because                                                                   a) Solar energy is renewable source of energy and hence can be obtained continuously.                                                               b) Being the poor country Nepal can't afford higher prices of fuels and gases.
18.  Write any two causes of energy crisis.                                                                                                             Ans: Causes of energy crisis are:                                                                                                                                          a) Lack of development and use of alternative or renewable energy sources.                                                                               b) Over use of non-renewable source of energy like petroleum products.                                                                             c) High demand of energy sources due to population growth.
d)  Due to unplanned unmanaged urbanization and industrialization.
19.  What is biomass?                                                                                                                                                                    Ans: The waste materials of living organism (like cattle dung) and dead part of living beings (like wood, tree, sawdust, straw) are called biomass.
20.  What is LPG? Write its composition and use.                                                                                                     Ans: A gas liquefied by compression, consisting of flammable hydrocarbons, as propane and butane, obtained as a by-product from the refining of petroleum or from natural gas is called Liquefied Petroleum Gas (LPG).
Compositions of LPG are:
a)     Ethane
b)     Propane
c)     Butane
It is used as domestic fuel.
21.  What type of energy is coal? Why it is called fossil fuel?                                                                                    Ans: Coal is non-renewable source of energy.
Coals are formed from the fossilization of dead organism under the soil from millions of years. So, it is called fossil fuel.
22.  Why more preference is given for the development of alternate source of energy in the world? Give two reasons.                                                                                                                                    Ans: More preference is given for the development of alternate source of energy in the world because                                                            a) To reduce the rate of consumption of non-renewable source of energy like petroleum products.                                                                   b) To prevent from the energy crisis in the near future.
23.  What is fossil fuel?                                                                                                                                  Ans: The fuels which are presented in the earth crust as remains of animals and plants over a period of millions of years are called fossil fuel.                                                                                                                                                                                                   For instant: Coal, Petrol, Natural gas, Mineral oil, etc.
24.  Why higher priority should be given for the production of hydroelectricity in the context of Nepal? Give two reasons.                                                                                                                             Ans: Higher priority should be given for the production of hydroelectricity in the context of Nepal because:                                                                                                                                                     a) Nepal has many fast flowing rivers and perennial rivers so generation of hydroelectricity will be easy.                                                             b) Hydroelectricity is cheaper, long term basis, renewable and pollution free source of energy.                                                                        c) In Nepal there is lack of mineral and petroleum substances.
25.  What is geothermal energy? Write its use.                                                                                                          Ans: The energy which is obtained from heat of earth core is called geothermal energy.
Uses of geothermal energy are:
a) It is used to warm the room in cold region.
b) It is used to generate electricity.

26.  Differences between nuclear fusion and fission?                                                                                                    Ans: Differences between nuclear fusion and fission
 S.N Nuclear fusion S.N Nuclear fission a) In a nuclear fusion, two light nuclei combine to form a heavy nucleus. a) In a nuclear fission, a heavy nucleus splits into two lighter nuclei. b) Nuclear fusion reactions have not been controlled so far. b) Nuclear fission reactions have been controlled to generate electricity.

27.  Give any two reasons that 'The sun is considered as the main source of energy.'
Ans: 'The sun is considered as the main source of energy' because
a) The sun provides the energy for all living beings on the earth directly or indirectly. We use solar energy for cooking foods, drying cloths and generating electricity. Coal, mineral, oil etc. are the outcome of the solar energy. Plant stores solar energy in the form of chemical energy. It means that plants use solar energy for the process of photosynthesis.
b) Hydro power is also an outcome of solar energy. Water in the river is either formed by melting of ice or raining. For the melting of ice, solar energy is needed. Similarly, for raining water of river and ocean must be evaporated.
28.  What is nuclear fuel?
Ans: The energy which is obtained by nuclear fusion reaction and nuclear fission reaction in atomic power-plants is called nuclear fuel.
29.  Give two reasons to justify that the use of hydroelectricity should be increased more than that of fossil fuel energy.
Ans: The use of hydroelectricity should be increased more than that of fossil fuel energy because
a)    Hydropower is renewable source of energy but fossil fuels are non-renewable source of energy.
b)    Hydropower is pollution free source of energy but fossil fuel cause air pollution when they are used.
30.  What do you meant by alternative source of energy? Give any three alternative source of energy.
Ans: The source of energy which are used instead of non-renewable source of energy are called alternative source of energy.
For e.g.: Hydropower, Solar energy, Geothermal power, Wind energy, Tidal energy, Biomass energy etc.
31.  What is thermonuclear fusion reaction?
Ans: The reaction in which two light nuclei of light atom combine together to form a heavy nucleus with the release of tremendous amount of heat energy during nuclear reaction is called thermonuclear fusion reaction.
32.  Fossil fuel is known as non-renewable source of energy, why?
Ans: Fossil fuel is known as non-renewable source of energy because they have limited stock in nature. Once it is used up we cannot use again & again and takes millions of year to reproduce.
33.  What are the conditions of nuclear fusion reaction found in the sun?
Ans: The conditions of nuclear fusion reaction found in the sun are:                                                                                                                     a) There is large amount of hydrogen gas.                                                                                                                                                      b) There is high temperature and high pressure.                                                                                                       c) There is huge amount of helium gas.
34.  Differences between thermonuclear fusion and fission?
 Thermonuclear fission Thermonuclear fusion It is chain reaction. It is not chain reaction. It can be controlled. It cannot be controlled.

35.  What is non-renewable energy?                                                                                                                                                                       Ans: The energy which cannot be reproduced in short time and cannot be used continuously for long period of time is called renewable energy.                                                                                                                                                                                                                     For instant: Fossil fuel (like coal, petroleum), Natural gases, Nuclear energy, etc.
36.  Clarify the mass-energy relation proposed by Albert Einstein.
Ans: the equation is famous equation given by Albert Einstein in his theory of relativity in 1905. This equation shows that mass of a body can be transferred into energy and vice versa. It is denoted by E = mc2 where,
E = energy, m = mass and c = velocity of light.
This equation E = mc2 is used to calculate the energy produced during the transformation of mass into energy.
37.  What are the three conditions required in the sun to produce enormous energy?                                                                 Ans: Any three conditions required in the sun to produce enormous energy are mentioned below:                                                                            (i) In the sun, there is sufficient amount of hydrogen gas.                                                                                                                                     (ii) There is presence of necessary temperature in the sun for formation of free protons.                                                                                (iii) There is presence of necessary pressure for combination of free protons.
38.  Write the uses of hydropower.
Ans: The uses of hydropower are:
a)     It is used for generating electricity.
b)     It has multipurpose use i.e. it is used to run many types of instruments.
39.  Write down the advantages and disadvantages of nuclear energy.
Ans: Advantages of nuclear energy:
a) It is used to generate electricity.
b) It is used to treat diseases like cancer.
c) It can be used as the alternate source of energy.
Disadvantages of nuclear energy:
a) It causes serious health hazards due to the emission of radiation like alpha, beta, gamma etc.
b) Nuclear energy is very expensive to establish.
c) It pollute environment badly.
40.  Differences between renewable and non-renewable source of energy.
Ans: Differences between renewable and non-renewable source of energy are.
 S.N Renewable source of energy S.N Non-renewable source of energy a) This type of source of energy can be reproduced in short time. a) It takes millions of years to reproduce them. b) They are present in great amount. b) They have limited stock in nature. c) This type of source of energy is never depleted. c) This type of source of energy gets depleted.

41.  Give the name of source of energy produced by nuclear fission.
Ans: The source of energy produced by nuclear fission reaction is nuclear energy.

Heat
1.   There is gap between the tracks of rail, why?
Ans: The metals expand or contract with the changes in temperature. The expansion or contraction depends on the coefficient of thermal expansion of the material. Based on this property of the metal, the gap in between the tracks of rail is provided. If there weren't gaps, the metal would buckle or develop stress fractures as it expanded and contracted.
Or
The metals expand or contract with the changes in temperature. When the trains move on the track with a speed, heat is produced due to friction as a result the volume of railway track increases, if the gap is not left, it bends and accident may happen. So, to avoid such accident gap between the tracks of rail is made.
2.   Name the instrument which is made on the basis of expansion of heat.
Ans: Thermometer is the instrument which is made on the basis of expansion of heat.
3.   What does it mean by the specific heat capacity of a substance is 140j/kg0C?
Ans: The specific heat capacity of a substance is 140j/kg0C. It means that 140j of heat energy is required to raise the temperature of 1Kg of a substance by 10C.
4.   What is specific heat capacity?
Ans: The amount of heat energy required to raise the temperature of 1Kg of a substance by 10C or 1K is called specific heat capacity.
5.   On what factors does the quantity of heat of an object depend?
Ans: The quantity of heat of an object depends on:                                                                                                                                           a) Mass of an object.                                                                                                                                                                                         b) Specific heat capacity of an object.                                                                                                                                                              c) Change in temperature of an object.
6.   Why does specific heat capacity of objects differ?
Ans: Specific heat capacity of objects differs because objects have different velocity of molecules and different molecular arrangement as well as different heat retaining capacity.
7.   Why is the density of water maximum at 40C?
Ans: When water is cooled up to 40C it contracts maximum i.e. its volume is minimum at 40C and when cooled below 40C its volume is increased. Therefore, water has maximum density at 40C.
8.   Two objects 'x' and 'y' have equal mass. Object 'x' has more specific heat capacity than object 'y', if both are given equal amount of heat, which object has more temperature and why?
Ans: If both objects 'x' and 'y' are given equal amount of heat, object 'y' would have more temperature because the specific heat capacity of objects 'y' is less than that of objects 'x' i.e. less amount of heat is sufficient to raise the temperature of objects 'y' by 10C.

9.   Why is only water used to cool the engine of vehicle?
Ans: Water has the highest specific heat capacity (i.e. 4200J/Kg0C) among all the liquid. Due to its high specific heat capacity, it observes more heat energy from the hot engines without appreciable rise in temperature of it. As a result, the cooling of engines is more effective with water than with any other liquid. Hence, only water is used to cool the hot engine of vehicle.
10.  Write the relationship between heat gain or loss by an object with mass.
Ans: The amount of heat gain or loss by an object is directly proportional to the mass of an object.
i.e. Q = msdt
Or, Q Î± m
11.  What is anomalous expansion of water? Write its one advantage.
Ans: Generally, volume of liquids increases on heating but when water at 00C is heated, its volume decreases up to 40C and begins to increase after 40C only. This unusual property shown by the water from 00C to 40C is known as anomalous expansion of water.
Its one advantage is
a)     Aquatic animals can stay alive in a frozen pond.
12.  The boiling temperature of water cannot be measured with the help of an alcohol thermometer, why?
Ans: The boiling point of water is 1000C while the boiling point of the alcohol is only 780C. Since the boiling point of the water is more than that of the alcohol, therefore the boiling temperature of water cannot be measured with the help of an alcohol thermometer.
13.  Write the application of high specific heat capacity of water.
Ans: The applications of high specific heat capacity of water are:
a)    Water is used to cool the engine of vehicles as it observes more heat from it and makes cooling effective.
b)    Water is used in hot bag because water can obtained more heat energy.
c)    Cold water is used to lower down the high fever by keeping wet handkerchief on forehead of the patient.
14.  Define heat according to molecular theory? Write C.G.S. unit of the heat.
Ans: The total kinetic energy possessed by all the molecules of a body is called heat.
C.G.S. unit of the heat is calorie.
15.  The water pipes in very cold places are burst in winter, why?
Ans: In winter, when water trapped in a pipe cools to 40C, the pipe contains maximum amount of water. When the water cools further from 40C to O0C, it expands considerably. The expansion in volume of water in the pipe gives outward pressure in the pipe due to which the pipe bursts.
16.  Two beakers A and B are filled with water at 40C. What happens if beaker A is heated and beaker B is cooled down? Why?
Ans: If the breaker A is heated and beaker B is cooled, water overflows from both beakers. Because water has minimum volume at 40C. Therefore, when the temperature of water rises from 40C, it expands and overflows. Similarly, when the temperature decreases from 40C, the water again expands and overflows.
17.  What is specific heat capacity of water?
Ans: The specific heat capacity of water is 4200J/Kg0C.
18.  How much joule is there in 1 calorie?
Ans: 1 calorie = 4.2 Joules
19.  At what temperature the density of water is maximum?
Ans: At 40C temperature the density of water is maximum.
20.  Write the differences between heat and temperature?
Ans: The differences between heat and temperature are:
 S.N Heat S.N Temperature a) It is the form of energy which gives the sensation of coldness and hotness. a) It is the degree of hotness and coldness of a body. b) Its S.I. unit is Joule (J). b) Its S.I. unit is kelvin (K). c) Calorimeter is used to measure it. c) A thermometer is used to measure it. d) It measures the total kinetic energy of all the molecule of a substance. d) It measures the average kinetic energy of the molecule of a substance.

21.  A beaker is filled with water. No single drop can be added. The temperature of water is 40C. What will happen if it is cooled? Explain with reason.
Ans: If the beaker is cooled, the water in the beaker will overflow because water at 40C has maximum density and lower volume. When a beaker filled with water at 40C is cooled, the density of water decreased and the volume of the water increased.

22.  The different in temperature is low near sea. Give reason.
Ans: Sea consist of large amount of water and specific heat capacity of water is very high i.e. 4200J/kg0C. So, water needs large amount of heat energy to increase the temperature. Similarly, water has to lose large amount of heat energy to decrease the temperature. Hence, water gets heated or cooled very slowly which makes the different in temperature low near sea.
23.  What do you mean by one calorie?
Ans: The amount of heat energy required to raise the temperature of 1g of pure water by 10C or 1K is called 1 calorie.
24.  Write two reasons for using mercury as thermometric liquid.
Ans: Reasons for using mercury as thermometric liquid are:
a) It is a good conductor of heat.
b) It has low specific heat capacity i.e. 460 J/Kg0C.
c) It is opaque and shining.
25.  What is heat equation? Write the formula of heat equation?
Ans: The product of mass, specific heat capacity and change in temperature is equal to the heat gained or lost by an object. It is called heat equation.
Heat gained or lost = mass specific heat capacity  change in temperature
i.e. Q = m s dt
26.  Study the following table
 Metal Specific heat capacity X Y Z 900 J/Kg0C                    460 J/Kg0C                      380 j/Kg0C

a)    What is mean by the specific heat capacity of metal Y is 460j/kg0C?
Ans: The specific heat capacity of metal Y is 460j/kg0C. It means that 460j of heat energy is required to raise the temperature of 1Kg of metal Y by 10C.
b)    Different metal balls with equal mass are heated to 1000C and put on the wax-slab, which metal ball will penetrate most deep and why?
Ans: Metal ball X will penetrate most deep on the wax-slab when different metal balls with equal mass are heated to 1000C because the specific heat capacity of metal ball X is more than that of others metal balls.
c)    Which above substance will gain more temperature while heating equal mass of all the three substance supplied with equal amount of heat? Give reason.
Ans: Metal Z will gain more temperature while heating equal mass of all the three substance supplied with equal amount of heat because the specific heat capacity of metal Z is less than that of other metals i.e. less amount of heat is sufficient to raise the temperature of metal Z by 10C.
27.  Well water is warmer in the morning during winter season, why?
Ans: The specific heat capacity of soil (land surface) is very less as compared to that of water. So, during winter night, the temperature of soil decreases very fast than that of water. As a result, the temperature of water inside the well is higher than the surrounding, this makes well water warmer in the morning during winter season.
28.  What effect will be seen on the density and the volume of water when it is heated from 00C to 100C.? What is this type of behavior called? How is this type of behavior of water useful for aquatic life?
Ans: When water is heated from 00C to 100C, it contracts up to 40C because the density of water increased and volume of water decreased. Similarly, when it is heated more than 40C it expands because the density of water decreased and volume of water increased.
Such type of behavior of water is called anomalous expansion of water.
In cold countries, fishes and other aquatic creatures survive in the pond due to the anomalous expansion of water even through surface of the pond has frozen to ice.
29.  Write the two factor that affect heat energy.
Ans: The two factors that affect heat energy are:
a)    Mass of an object.
b)    Average kinetic energy of the molecules of an object.

Light
1.   Write two differences between converging lens and diverging lens.
Ans: Differences between converging lens and diverging lens are:
 S.N Converging (convex) lens S.N Diverging (concave) lens a) It is thicker at the middle and thinner at the edges. a) It is thinner at the middle and thicker at the edges. b) It converses parallel beam of light. b) It diverges parallel beam of light. c) It has a real focus. c) It has a virtual focus.

2.   Write two differences between telescope and compound microscope.
Ans: Differences between telescope and compound microscope are:
 S.N Telescope S.N Compound microscope a) It is used to see distant object clearly. a) It is used to see very tiny object clearly. b) The focal length of the objective is longer than that of the eyepiece. b) The focal length of the objective is shorter than that of the eyepiece. c) The final image is formed at infinity. c) The final image is formed beyond the objective lens.

3.   Write two differences between near point and far point.
Ans: Differences between near point and far point are:
 S.N Near point S.N Far point a) The nearest point from the eye up to which an object can been seen clearly is called near point. a) The farthest point from the eye up to which an object can been seen clearly is called far point. b) For a normal eye, near point is 25cm. b) For a normal eye, far point is at infinity.

4.   What is the function of cillary muscles and Iris?
Ans: The function of
a)    Cillary muscles: - It controls the lens of the eye or changes the shape of the lens for near and distant vision.
b)    Iris: - It controls the amount of light entering the eye.
5.   What is power of lens? Write its S.I. unit.
Ans: The capacity of a lens to converge or diverge rays of light falling on it is called power of lens.
Its S.I. unit is dioptre (D).
6.   Why concave lens is called diverging lens?
Ans: When a parallel beam of light is incident on a concave lens, it diverge the parallel beam of light from a point after refraction through the lens. Because of this diverging action concave lens is called diverging lens.
7.   Define focal length of a lens. Draw suitable diagrams show the focal length.
Ans: The distance between the optical centre and the principle focus of a lens is called focal length of a lens. It is denoted by f.

8.   In what condition the image formed by a convex lens is erect?
Ans: When an object is placed between focus and optical centre of convex lens, the image formed by the lens is erect.
9.   A student uses spectacles of power -2.5D. What type of lens is used in the spectacles? What type of defect of his eye?
Ans: A student uses spectacles of power -2.5D. It means a student is using concave lens in the spectacles.
The defect of his eye is short sightedness (Myopia).
10.  Write two differences between lens of eye and lens of camera.
Ans: Differences between lens of eye and lens of camera are:
 S.N Lens of eye S.N Lens of camera a) Its focal length is variable i.e. focal length changes. a) Its focal length is fixed. b) It is made up of protein and crystalline in nature. b) It is made up of glass.
11.  Draw a neat and labeled ray diagram showing the image formed by a convex lens when an object is placed. Also, write down the characteristics of the image formed.
(a) At 2F            (b) beyond 2F
a) When an object is placed at 2F.
Characteristics of the image formed:
i)  The image is formed at 2F on the other side of the object.
ii)  The image is real, inverted and of same size as the object.
Note: This types of image formation is used in terrestrial telescope;

b)  When an object is placed beyond 2F.
Characteristics of the image formed:
i)    The image is formed between F and 2F.
ii)   The image is real, inverted and diminished (i.e. smaller than the object)
Note: This type of image formation is used in photographic camera.
12.  What type of lens is used to treat short sightedness? Why? Explain with ray diagram.
Ans: Concave lens is used to treat short sightedness because in short
sightedness image is formed before the retina. As the concave lens
diverges light rays coming from the object which are further converge
by the crystalline lens so the focal length increases by the combination
of two lenses. As a result image is formed in retina.
13.  Write formula of power and magnification of lens.
Ans: Formula of
a)    Power of lens =  1/focal length of lens (in meter)
i.e. p =1/f
b)   Magnification of lens =   height of the image (I)/height of the object(o)                                                                          =image distance(v)/ object distance(u)
i.e. m = I/O = v/u
14.  Focal length of a convex lens is 2 cm. An object of height 1 cm is placed at a distance of 1cm from the lens. Draw a ray diagram and write the nature of image.
Ans: The image formed by a convex lens of focal length 2 cm when an object is
placed 1cm away from the lens is shown below:

Characteristics of the image formed:
a)     The image is virtual, erect and magnified.
b)     The image is formed behind the object on the same side of the lens.
15.  What is near point of an eye?
Ans:  The nearest point from the eye up to which an object can been seen clearly is called near point.
16.  What do you mean by defeat of vision?
Ans: An eye which does not form the image of an object lying between the near point and far point on the retina is said to be suffer from a defect of vision.
17.  Write the causes of short sightedness.
Ans: Causes of sightedness are:
a)    The eye ball gets elongated.
b)    The focal length of eye lens decreases.
18.  What is magnification of lens?
Ans: The ratio of the height of the image to the height of the object is called magnification of lens.
19.  A person can only read the newspaper at far distance from his eyes. What kind of vision defect has he got?
Ans: A person can only read the newspaper at far distance from his eyes. He has long sightedness.
20.  Write the remedy of short sightedness.
Ans: Short sightedness can be removed by putting a concave lens of suitable focal length in front of the eye.
21.  Why convex lens is called converging lens?
Ans: When a parallel beam of light is incident on a convex lens, it converge the parallel beam of light at a common point after refraction through the lens. Because of this converging action convex lens is called converging lens.
22.  Write an application of convex and concave lens.
Ans: The application of
A)   Convex lens
Uses: - i)    It is used in spectacles to correct the long sightedness.
ii)    It is used to make simple microscope, compound microscope, camera, telescope etc.
iii)   It is used as magnifying glass by watch makers and jewellers to see fine details.
B)    Concave lens
Uses: - i)     It is used in spectacles to correct the short sightedness.
ii)    It is used in doors to see the wider view of the person outside the doors.
iii)   It is used as an eye lens for Galilean telescope.
22.  Write an example of optical instrument.
Ans: Examples of optical instrument are:
a)    Camera
b)    Microscope
c)    Telescope
d)    Film projectors
23.  Write the causes of long sightedness.
Ans: causes of long sightedness are:
a)    The eye ball gets contracted.
b)    The focal length of eye lens increases.
24.  If the man with long sighted eye wants to read a textbook, what should be the distance between book and the lens? Give reason.
Ans: If the man with long sighted eye wants to read a textbook, the distance between book and the lens should be more than 25cm because a person suffering from long sightedness can see the distant object clearly but cannot see the nearby object clearly.
25.  A man used a lens of power 1.5D. What type of defect of his eye? What type of lens is used in the spectacles?
Ans: A man used a lens of power 1.5D. It means he has long sightedness.
He should use a convex lens in his spectacles.
26.  What is short sightedness and long sightedness?
Ans: The defect of vision in which a person can see the nearby object clearly but cannot see the distant object clearly is called short sightedness.
The defect of vision in which a person can see the distant object clearly but cannot see the nearby object clearly is called long sightedness.
27.  Write the remedy of long sightedness.
Ans: Long sightedness can be removed by putting a convex lens of suitable focal length in front of the eye.
28.  After examining the eye of the student, a doctor suggested him to use the spectacles of power -1.5D. What type of defect is there in his eye?
Ans: After examining the eye of the student, a doctor suggested him to use the spectacles of power -1.5D. It means he has short sightedness.
29.  What is 1 dioptre?
Ans: A lens is said to have power of 1 dioptre if its focal length is 1m.
30.  What type of lens is used to treat long sightedness? Why?
Ans: Convex lens is used to treat long sightedness because in long sightedness image is formed after the retina. As the convex lens converges light rays coming from the object which are further converge by the crystalline lens so the focal length decreases by the combination of two lenses. As a result image is formed in retina.
31.  Why is photographic film developed in dark room?
Ans: The film used in photography is coated with silver bromide (AgBr) which is very sensitive to light. So, photographic film is developed in dark room.
32.  Draw a neat diagram showing myopia.