Question

When monochromatic red light is used instead of blue light in a convex lens, its focal length will :

• A. increase
• B. decrease
• C. remain same
• D. does not depend on colour of light

Answer 1

Answer: (A)

increase

Answer 2

$\mu_{R} < \mu_{B}$ $\frac{1}{f} = \left(\mu-1\right) \left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)$ $\frac{1}{f_{B}} > \frac{1}{f_{R}}$ $f_{R} > f_{B}.$