Question

A spherical mirror forms a real image of a point object placed in front. The distance of the image and object from the mirror is 30 cm and 0.2 m respectively.The focal length and nature of the mirror are

• A. 120 mm, concave
• B. 120 mm, convex
• C. 60 mm, concave
• D. 60 mm, convex

Answer 1

The correct option is C 60 mm, concave

$\begin{array}{c}u=\text{Distance of object}=20cm\\ v=\text{Distance of image}=30cm\\ \text{Mirror formula}=\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\\ =\frac{1}{-30}+\frac{1}{20}=\frac{1}{f}\\ \frac{20-30}{600}=\frac{1}{f}\\ f=-60cm\\ \text{- sign means concave}\\ f=60cm\end{array}$