Ray Optics and Optical Instruments - Result Question 40
41. A body is located on a wall. Its image of equal size is to be obtained on a parallel wall with the help of a convex lens. The lens is placed at a distance ’ $d$ ’ ahead of second wall, then the required focal length will be
[2002]
(a) only $\frac{d}{4}$
(b) only $\frac{d}{2}$
(c) more than $\frac{d}{4}$ but less than $\frac{d}{2}$
(d) less than $\frac{d}{4}$
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Answer:
Correct Answer: 41. (b)
Solution:
- (b) Using the lens formula $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
Given $v=d$, for equal size image $|v|=|u|=d$
By sign convention $u=-d$
$\therefore \frac{1}{f}=\frac{1}{d}+\frac{1}{d} \quad$ or $f=\frac{d}{2}$
