Question

Focal length of a convex lens will be maximum for:

• A. Red light
• B. yellow light
• C. Green light
• D. blue light

Answer 1

Answer: (A)

Red light

Answer 2

From the relation $f\propto \frac{1}{(\mu -1)}$ and also $\mu \propto \frac{1}{\lambda }$ Therefore, $f\propto \lambda$ We know that ${{\lambda }_{r}}>{{\lambda }_{v}}$ Therefore, focal length will maximum for red light.