A concave mirror is immersed in water. What happens to its focal length?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → It remains the same.
Option (1) It is halved. — Incorrect. The radius of curvature \( R \) does not change in water, and \( f=R/2 \) contains no refractive index, so nothing can halve the focal length.
Option (2) It is doubled. — Incorrect. The focal length of a lens changes in water. This is because refraction depends on the refractive index of the glass relative to the surroundings. A mirror does not refract the light, so its focal length is not increased.
Option (3) It remains the same. — Correct. In water, the rays still obey the law of reflection at the same curved surface. So they meet at the same focus, and \( f=R/2 \) is unchanged.
NCERT (2026-27):
Focal Length of Spherical Mirrors – “The distance between the focus F and the pole P of the mirror is called the focal length of the mirror, denoted by f. We now show that f = R/2, where R is the radius of curvature of the mirror.”