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)

From lens makers formula $\frac{1}{f}=(\mu -1)\left(\frac{1}{R_1}-\frac{1}{R_2}\right)$ $f\propto \frac{1}{(\mu -1)}$ $f\propto \frac{1}{\mu }$ $\Rightarrow$ $\mu \propto \frac{1}{\lambda }$ So, $f\propto \lambda$ But wavelength of red light is more as compared to other given light. Hence, focal length of convex lans is maximum for red light.