Question

An object is at a distance of 10cm from a mirror and the image of the object is at a distance of 30cms from the mirror on the same side as the object. Then the nature of the mirror and its focal length is

• A. convex, 7.5cm
• B. concave, 7.5cm
• C. convex, 15cm
• D. concave, 15cm

Answer 1

Answer: (D)

concave, 15cm

$\begin{array}{c}u=-10cm\\ v=-30cm\\ f=?\\ \frac{1}{v}-\frac{1}{u}=\frac{1}{f}\\ \frac{1}{-30}+\frac{1}{10}=\frac{1}{f}\\ \frac{10-30}{-300}=\frac{1}{f}\\ f=-15cm\\ \text{-ive sign denotes it}\\ \text{is convace mirror.}\end{array}$