Laws of reflection:-
(i) The angle of incidence is equal to the angle of reflection,
and
(ii) The incident ray, the normal to the mirror at the point of
incidence and the reflected ray, all lie in the same plane.
A spherical mirror, whose reflecting surface is curved inwards,
that is, faces towards the centre of the sphere, is called a concave mirror. A
spherical mirror whose reflecting surface is curved outwards, is called a
convex mirror.
The centre of the reflecting surface of a spherical mirror is a
point called the pole.
The centre of curvature is not a part of the mirror. It lies
outside its reflecting surface. The centre of curvature of a concave mirror
lies in front of it. However, it lies behind the mirror in case of a convex
mirror.
For spherical mirrors of small apertures, the radius of
curvature is found to be equal to twice the focal length. We put this as R = 2f. The principal focus of a spherical mirror lies midway between
the pole and centre of curvature.
The focal length of a spherical mirror is equal to half its
radius of curvature.
Concave mirrors are commonly used in torches, search-lights and
vehicles headlights. They are often used as shaving mirrors to see a larger
image of the face. The dentists use concave mirrors to see large images of the
teeth of patients. Large concave mirrors are used to concentrate sunlight to
produce heat in solar furnaces.
Convex mirrors are commonly used as rear-view (wing) mirrors in
vehicles.
Light does not travel in the same direction in all media. It
appears that when travelling obliquely from one medium to another, the
direction of propagation of light in the second medium changes. This phenomenon
is known as refraction of light.
Light travels in vacuum with an enormous speed of 3×108 m s-1.
The following are the laws of refraction of light.
(i) The incident ray, the
refracted ray and the normal to the interface of two transparent media
at the point of incidence, all lie in the same plane.
(ii) The ratio of sine of angle of incidence to the sine
of angle of refraction is a constant, for the light of a given colour and for
the given pair of media. This law is also known as Snell’s law of refraction.
The refractive index of a transparent medium is the ratio of the
speed of light in vacuum to that in the medium.
An imaginary straight line passing through the two centres of
curvature of a lens is called its principal axis. The central point of a lens
is its optical centre. It is usually represented by the letter O. A ray of
light through the optical centre of a lens passes without suffering any
deviation. The effective diameter of the circular outline of a spherical lens
is called its aperture.
Several rays of light parallel to the principal axis are falling
on a convex lens. These rays, after refraction from the lens, are converging to
a point on the principal axis. This point on the principal axis is called the
principal focus of the lens
Several rays of light parallel to the principal axis are falling
on a concave lens. These rays, after refraction from the lens, are appearing to
diverge from a point on the principal axis. This point on the principal axis is
called the principal focus of the concave lens.
The distance of the principal focus from the optical centre of a
lens is called its focal length. The letter f is used to represent the
focal length.
According to the convention, the focal length of a convex lens
is positive and that of a concave lens is negative.
Lens formula is expressed as:- 1/v-1/u-1/f
The ability of a lens to converge or diverge light rays depends
on its focal length. The power of a lens is defined as the reciprocal of its
focal length. It is represented by the letter P.
Next Topic:-Science Class 10 NCERT Textbook- Lesson-11
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