Question

An illuminated object lies at a distance 1.0 m from a screen. A convex lens is used to form the image of object on a screen placed at distance 75 cm from the lens. Find : (i) the focal length of lens, and (ii) the magnification.

• A. (i) 15.75 cm (ii) -2
• B. (i) 18.75 cm (ii) -3
• C. (i) 19.75 cm (ii) 2
• D. (i) 17.75 cm (ii) -3

Answer 1

Answer: (B)

(i) 18.75 cm (ii) -3

$\begin{array}{c}u=25cm\\ v=75cm\\ \frac{1}{v}+\frac{1}{u}=\frac{1}{f}\\ \frac{1}{75}-\frac{1}{25}=\frac{1}{f}\\ \frac{1+3}{75}=\frac{1}{f}\\ f=+\frac{75}{4}\end{array}$

$\begin{array}{c}=18.75cm\\ \text{Magnification}=\frac{v}{u}=\frac{75}{-25}=-3\text{Ans}\end{array}$