Question

The focal length of a spherical mirror is:
A) Maximum for red light
B) Maximum for blue light
C) Maximum for white light
D) Same for all lights

Answer 1

Hint: To solve this question we have to be familiar with spherical mirrors and some basic terms. The distance from the pole to the focal point is the focal length (f).

Complete step by step solution:
First, we must understand the few important terms related to spherical mirrors. a mirror, whose reflecting surface is a part of a sphere, is called a spherical mirror. There are different terms associated with the spherical mirror. First, we look into these terms. There is a centre on the reflecting surface of the mirror which cuts the mirror equally half. This geometric centre of the reflecting surface of the spherical mirror is known as the ‘Pole’ of the mirror. It is represented by ‘P’. The whole area which is available for reflection is the aperture of the mirror.

We already discussed that a spherical mirror is a part of a complete sphere. For that sphere there will be a centre. This centre of the sphere is also a centre of the spherical mirror. This centre is known as the centre of curvature of spherical mirrors. The distance between the centre of spherical mirror and the pole is known as radius of curvature. The imaginary line which joins the centre of curvature and the pole is the principal axis.

Now, we consider a beam of parallel light rays incident on a spherical mirror that, after reflection, converges to a fixed point on the principal axis. This fixed point is called principal focus. The distance between this focus and the pole is known as Focal length.
Focal length is the property of a spherical mirror. It does not depend on the light used. So, it is the same for all lights.

Hence, the correct answer is the focal length of a spherical mirror is the same for all lights.

Note: The focal length of a mirror or lens is Independent of its wavelength and hence it is also independent of its color. So it is the same for all light.