A concave lens and a convex mirror both are dipped in an oil of refractive index 1.2. The focal length of:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → lens will change but of mirror will remain same
The focal length of a lens depends on the refractive index of the lens material relative to the surrounding medium, while that of a mirror is fixed by geometry alone.
\( \displaystyle \frac{1}{f} = \left(\frac{\mu_{\mathrm{lens}}}{\mu_{\mathrm{medium}}} - 1\right)\left(\frac{1}{R_1} - \frac{1}{R_2}\right) \)
Changing the medium from air to oil changes \( \mu_{\mathrm{medium}} \), so \( f \) of the lens changes. For a mirror, reflection takes place at the surface and \( f = R/2 \), which involves only the radius of curvature, so the medium makes no difference.