SEE Science (Light) Notes

Light

1. Write two differences between converging lens and diverging lens.

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.

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.

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 jewelers 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.