Reflection by Spherical Mirrors - NEET Physics Questions

Question 21: easy

Assertion (A): A concave mirror and a concave lens have the same focal length in air. When dipped in water, the focal length of the two are equal.

Reason (R): The focal length depends only on the radii of curvature.

1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)

2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)

3. (A) is true but (R) is false

4. Both (A) and (R) are false

View Answer

The focal length of a mirror \(f_m = R/2\) depends only on its radius of curvature and is independent of the surrounding medium. The focal length of a lens \(1/f_l = (n_l/n_m - 1)(1/R_1 - 1/R_2)\) depends on the refractive indices of the lens material (\(n_l\)) and the surrounding medium (\(n_m\)), as well as radii of curvature. Thus, Assertion (A) is false as their focal lengths are generally not equal and lens focal length changes with medium. Reason (R) is false as lens focal length also depends on refractive indices.

💬 Discuss this Question