Question

Violet light is incident on a converging lens of focal length f. State with reasons how focal length of the lens will change, if the violet light is replaced by red light.

Answer 1

The focal length of a lens is given by

\(\frac{1}{f} = (\mu - 1)(\frac{1}{R_1 } -\frac{1}{R_2})\)

Since µ_r < µ_v

∴ \(\frac{1}{f_r} < \frac{1}{f_v}\)

or f_r > f_v

Thus, the focal length will increase.