Definitions (Light)
LIGHT
Lens:
a lens is a piece of transparent glass bounded by two refracting surfaces which
are usually spherical.
Convex Lens: a lens that is thicker at its center than at edges is called convex
lens.
Concave lens: The lens which is thicker at its edges
than at the middle is called concave lens.
Center of curvature: The center of the spherical surface
whose parts forms the lens is called center of curvature.
Radius of curvature: The radius of a sphere whose surface forms
the lens is called radius of curvature.
Principle axis: It is a straight line passing through
the center of curvature.
Optical center: The geometrical center of a lens is
called its optical center.
Principal focus: It is the point on the principle axis
where parallel rays of light meets or seems to meet after refraction through
the lens.
Focal length: It is the distance between optical
center and center of curvature.
Real image: The image which can be obtained on the screen is called real
image.
Virtual image: Those images which cannot be obtained on
the screen are called virtual image.
Magnification:
The magnification of a lens is defined as the ratio of height of the
image to the height of the object.
Power of lens: It is defined as the reciprocal of focal
length of lens expressed in meter.
One Dioptre: A lens is said to be of 1 Diopter if its focal length is 1 m.
Optical instruments: Are those instruments that use mirror,
lens and prism.
Simple camera: A Simple camera is an optical instrument
that is used or taking photographs of object and people.
Eye:
Eyes are natural optical instrument which are concerned with the sense of
vision.
Near point: The nearest point, up to which an object can be seen clearly by
a human eye, is called near point.
Far point: The farthest point, up to which an object can be seen clearly
by human eye is called far point.
Power of Accommodation: The ability of an eye to change the
focal length of the eye lens so as to obtain the image on the retina is called
power of accommodation.
Persistence of vision: it is effect of vision due to which the
impression of an image last on the retina even after the removal of the object.
Defect of vision: An eye which does not form the image on
an object lying between the near point and far point on the retina is said to
suffer from defect of vision.
Long-sightedness (Hypermetropia): it is the defect of
vision which the eye cannot see the nearby object clearly but can see distant
object clearly.
Blind spot: The region of the retina where the optic nerves enters the eye
is called blind spot.
Yellow spot: The region in the center of the retina which has maximum concentration
of rods and cones is called yellow spot.
Short-sightedness (Myopia): It is the defect of vision in which the
eye can see near object clearly but cannot see distant object is called
Short-sightedness.
Microscope: It is an optical instrument which is used to see the magnified
image of a small object placed very close to it.
Telescope: It is an optical instrument which is used to see the distant
objects clearly.
Aperture: Aperture of a lens is the maximum portion of the spherical
surface through which refraction takes place.
Focusing; Focusing is the process of adjusting the distance between the
lens and the screen to produce sharp and clear image.
Lens formula: A formula which gives the relationship
between the focal length, object distance and image distance is called lens
formula. The reciprocal of focal length of lens is equal to the sum of
reciprocal of object distance and image distance, is lens formula.
Photographic camera: It is an optical instrument which is
used to record a permanent image of a object on a photographic film.
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