Question

If the wavelength of light incident on a convex lens is increased, how will its focal length change?

Answer 1

Focal length varies directly with wavelength and as wavelength is inversely with refractive index, its also varies inversely with the wavelength .

Focal length increases with wavelength and decreases with refractive index .

As we all know that Cuachy's equation gives relationship between refractive

index and wavelength of light for a particular transparent material and again

lens formula gives a relationship between focal length and refractive index, so

eliminating refractive index term from both equation should give you a

mathematical relation between the wavelength and the focal distance.

n(λ) = B + C/λ²

Here B and C are coefficient determined for the material .

Hope it helps

Answer 2

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.