question_answer

What changes in the focal length of a (i) concave mirror and (ii) convex lens occur, when the incident violet light on them is replaced with red light?

Answer:

                (i) For a concave mirror, \[f=R/2\]and is independent of the wavelength of light. So focal length does not change when violet light is replaced with red light. (ii) For a convex lens, \[f\propto \frac{1}{\mu -1}\] As\[{{\mu }_{R}}<{{\mu }_{V}},\] so\[{{f}_{R}}>{{f}_{V}}\]. So focal length increases en violet light is replaced with red light.