Question

On which of the following factors does the focal length of lens depend?

• A. Size of the object
• B. Power of the light source
• C. Wavelength of the light
• D. Surrounding medium

Answer 1

The Correct Option is D

The focal length of a lens depends on the refractive index of the material of the lens and the medium in which it is placed. The refractive index changes when the surrounding medium changes, which in turn affects the focal length. This can be understood from the lens maker’s formula: \[ \frac{1}{f} = \left(\frac{\mu_{\text{lens}}}{\mu_{\text{medium}}} - 1\right) \left( \frac{1}{R_1} - \frac{1}{R_2} \right) \] Where:
\( f \) = focal length of the lens
\( \mu_{\text{lens}} \) = refractive index of the lens
\( \mu_{\text{medium}} \) = refractive index of the surrounding medium
\( R_1, R_2 \) = radii of curvature of the lens surfaces
As seen in the formula, a change in \( \mu_{\text{medium}} \) (surrounding medium) will change the focal length \( f \).

The correct option is (D): Surrounding medium