Choose the correct relation between focal length \( f \) and radius of curvature \( R \) of a convex mirror from the following:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \( f = +0.5R \)
For a spherical mirror the focal length is half the radius of curvature, \( f = R/2 \). With the Cartesian sign convention the incident light travels in the positive direction. The pole, the focus and the centre of curvature of a convex mirror lie behind the mirror, on the positive side, so both \( f \) and \( R \) are positive.
\( \displaystyle f = +\frac{R}{2} = +0.5R \)
NCERT (2026-27):
Summary – “… f is (approximately) half the radius of curvature R. f is negative for concave mirror; f is positive for a convex mirror.”