Chapter 24, Problem 1CQ

If a spherical mirror is immersed in water, does its focal length change? What if a lens is immersed in water?

To determine

What will happen to the focal length of a spherical mirror and lens, when it immersed in water.

Answer to Problem 1CQ

Spherical mirrors based on Law of reflection does not change focal length with medium. While lenses will change focal length because there will be change in refractive index with change in medium.

Explanation of Solution

Focal length is the distance between centres of the mirror to the focal point, where parallel rays converges.

Spherical mirrors are works on a principle of Law of Reflection. Law of reflection states that, angle of incidence is equal to angle of reflection and all lie in the same plane. Mirror will satisfy Law of reflection irrespective of the medium in which it is immersed. Thus, there is no change in focal length of spherical mirror.

However, lens works on the principle of refraction. As the medium changes, refractive index changes. Along with refractive index, focal length also changes. Higher the refractive index shorter will be the focal length.