Question

If the wavelength of incident light on a (i) concave mirror and (ii) convex lens is increased, how will the focal length of each of these change ?

Answer 1

If wavelength is increased for a

1. concave mirror : no change occurs as the focal length does not depend upon refractive index and wavelength

2. convex lens : when wavelength is increased,  speed of light increases hence the power will reduce and focal length will increase

Answer 2

The focal length of a convex lens increases as the wavelength increases. This is so because the refractive index decreases with an increase in wavelength of light.