Chapter 18, Problem 87A

To determine

To explain: The reason for Chromatic aberration to be a defect for lenses but not for mirrors.

Explanation of Solution

Introduction:

The focal length of a lens is governed by the refractive index of the material from which the lens is made up of. For the increase of wavelength, most of the material shows decrease in its refractive index. Hence there will be variation in the refractive index of the lens with the wavelength of light. Because of this variation in refractive index, it is not possible for a lens to focus light rays of all the wavelengths to a common focal point. This leads to a deception in an image of the object seen through a lens and is called chromatic aberration.

In lenses the light waves of different wavelengths travel with different speeds and hence gets refracted at slightly different angles. Due to this dispersion of light, it is not possible for a lens to focus light rays of all the wavelengths to a common focal point. Hence there will be a chromatic aberration in lenses.Mirrors reflects the light rays strikes on it and is independent of wavelength of incident light.