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

The correct option is A increase

The refractive index of a medium is inversely proportional to the wavelength of light used.
Thus, ${\mu }_R<{\mu }_B$
Now, $\frac{1}{f}=(\mu -1)(\frac{1}{R_1}-\frac{1}{R_2})$
Thus, $f_R>f_B$