Question

what changes will occur to the focal length of a a) concave mirror b) convex lens when the incident violet light is replaced by red colour?

Answer 1

In the case of mirror  the  Focal length is dependent on radius of curvature (f=R/2). So, no change in focal length for any colour light.

As per the Thin lens  approximation, focal length is not depending on the material of the lens. So, no change in focal length for any colour light.

Answer 2

Answer:

The refractive index of the material of a lens increases with the decrease in wavelength of the incident light. ... Thus, when we replace red light with a violet wavelength decrease and hence the focal length of the lens also decrease