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Light - Reflection and Refraction

Terms used in a Spherical Mirror

Light travels in a straight line and can change its direction when incident on a shiny surface.

Jatin looks inside a polished steel bowl and gets surprised to find his face appearing inverted inside the bowl. Furthermore, the image of his face changes its size as the bowl is moved towards or away from him. However, when he looks on the outer side of the same bowl, he finds his image to be erect. Why does this happen? This happens because the curved surface of the bowl acts as special kind of mirror, known as a spherical mirror. A spherical mirror can be made from a spherical ball.

 Take a tennis ball and cut it into two equal halves. The inner surface of each half is known as the concave surface, while the outer surface is called the convex surface.

There are two types of spherical mirrors

i) Concave mirrors

ii) Convex mirrors

Concave mirrors

A concave mirror is a spherical mirror whose reflecting surface is curved inwards. In a concave mirror, reflection of light takes place from the inner surface. This mirror resembles the shape of a ‘cave’. A Painted surface is a non-reflecting surface.

Convex mirrors

A convex mirror is a spherical mirror whose reflecting surface is curved outwards. In a convex mirror, the reflection of light takes place from its outer surface. A Painted surface is a non-reflecting surface.

Hence, the inward surface of the steel bowl or a spoon acts as a concave mirror, while its outer surface acts as a convex mirror.

There are some definitions associated with spherical mirrors, which will prove helpful in the discussion of spherical mirrors. But, before going into the definitions, let us understand the terms clearly.

So, the definitions of the terminologies are as follows:

Pole of a spherical mirror

The central point of the reflecting surface of a spherical mirror is termed as the pole. It lies on the mirror and is denoted by the letter (P).

Centre of curvature

The centre of curvature as the centre of a sphere from which the given spherical mirror (convex or concave) is obtained. It is denoted by the letter (C).

The distance between the centre of curvature and pole (PC) is known as the radius of curvature.

Principal axis of the spherical mirror

The imaginary straight line joining the pole (P) and the centre of curvature (C) is termed as the principal axis.

Focus

The focus (F) is the point on the principal axis of a spherical mirror where all the incident rays parallel to the principal axis meet or appear to diverge from after reflection.

For concave mirrors, the focus lies on the same side of the reflecting surface.

For convex mirrors, the focus is obtained on the opposite side of the reflecting surface by extrapolating the rays reflected from the mirror surface.

Radius of curvature (R) and the focal length (f) of a spherical mirror are related as

 R = 2f

Where, R is the distance between the centre of curvature and the pole of the mirror, while f is the distance between the focus and the pole of the mirror.

The focus of a spherical mirror always lies between the pole (P) and the centre of curvature (C).

Reflection by spherical mirrors

The different ways in which a ray of light is reflected from a spherical mirror are:

Case I: When the incident light ray is parallel to the principal axis.

In this case, the reflected ray will pass through the foc...

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