A concave mirror forms a sharp image of the Sun at a distance of 15 cm from the mirror. A candle is then placed 20 cm in front of the same mirror. The image of the candle is formed
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 60 cm in front of the mirror
Rays from the Sun reach the mirror parallel to each other. A concave mirror brings parallel rays together at its focus. So the image of the Sun lies at the focus, and \( f=-15\ \mathrm{cm} \).
\( u=-20\ \mathrm{cm} \)
\( \begin{aligned} \frac{1}{v} &= \frac{1}{f}-\frac{1}{u} \\ &= -\frac{1}{15}+\frac{1}{20} \\ &= -\frac{1}{60} \end{aligned} \)
\( v=-60\ \mathrm{cm} \)
A negative image distance puts the image on the same side as the candle. Hence, the image is formed 60 cm in front of the mirror.
NCERT (2026-27):
Focal length of spherical mirrors – “… when a parallel beam of light is incident on (a) a concave mirror … The reflected rays converge at a point F on the principal axis of a concave mirror … The point F is called the principal focus of the mirror.”
The mirror equation – “Dividing it by v, we get”
\( \displaystyle \frac{1}{v}+\frac{1}{u}=\frac{1}{f} \)\( \qquad (9.7) \)
“This relation is known as the mirror equation.”