Page 62 - Physics 10_Float

GEOMETRICAL OPTICS

This defect can be corrected with the aid of a suitable
converging lens. The lens refracts the light rays and they
converge to form an image on the retina. To an observer,
these rays appear to come from near point to form a sharp
virtual image on the retina (Fig.12.39-b).

SUMMARY

 When light travelling in a certain medium falls on the surface of another
medium, a part of it turns back in the same medium. This is called reflection of
light. There are two laws of reflection:
i. The incident ray, the reflected ray, and the normal all lie in the same plane.
ii. The angle of incidence is equal to the angle of reflection (i.e., i = r).
 Like plane surfaces, spherical surfaces also reflect light satisfying the two
laws of reflection.
 In mirrors, image formation takes place through reflection of light while in
lenses image is formed through refraction of light.
 The equation relating the distance of the object p from the mirror/lens,
distance of the image q and the focal length f of the mirror/lens is called
mirror/lens formula, given by 1 1 1
f = p + q
 Magnification of a spherical mirror or thin lens is defined as “the ratio of the
image height to the object height.” i.e., Image height h
Magnification m = = i
Object height h o
 Power of a lens is defined as “the reciprocal of its focal length in metres”. Thus Power
of a lens = P = 1 / focal length in metres. The SI unit of power of a lens is “Dioptre”,
denoted by a symbol D. If is expressed in metres so that 1 D = 1 m . Thus, 1 Dioptre f -1
is the power of a lens whose focal length is 1 metre.
 The refractive index ‘n’ of a material is the ratio of the speed of light ‘c’ in air to the
speed of light ‘v’ in the material, thus
Speed of light in air c
n = =
Speed of light in medium v
 The bending of light from its straight path as it passes from one medium into another
is called refraction.
 Refraction of light takes place under two laws called laws of refraction. These are
stated as:
i. The incident ray, the refracted ray, and the normal at the point of incidence
all lie in the same plane.

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