First slide

The lens

Question

An object has image thrice of its original size when kept at 8 cm and 16 cm from a convex lens. Focal length of the lens is

Moderate

A

8 cm
B

16 cm
C

Between 8 cm and 16 cm
D

Less than 8 cm

Solution

$m = \pm 3, using m = \frac{f}{f + u} For virtual image 3 = \frac{f}{f - 8} - - - - (i) For real image - 3 = \frac{f}{f - 16} .----(ii) Solving (i) and (ii) we get f = 12 cm$

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