Q. The focal distance of a plane mirror is
a) 2 cm
b) -1 cm
c) 1 cm
d) Infinity
Correct Answer : d) Infinity
Question from BPSC PT 71, 2025
Correct Answer: d) Infinity
Explanation:
– For mirrors, focal length f is related to the radius of curvature R by f = R/2.
– A plane mirror can be thought of as a spherical mirror with an infinite radius of curvature (R = ∞).
– Therefore, its focal length is f = ∞/2 = ∞.
– Physically, a plane mirror does not converge or diverge parallel rays; they remain parallel, so the focus is at infinity.
