Distance between an object and three times magnified real image is 40 cm. The focal length of the mirror used is ______ cm. [2026]

Question

Distance between an object and three times magnified real image is 40 cm. The focal length of the mirror used is ______ cm. [2026]

Smart Achievers · Accepted Answer

(4)

$m = - 3 = \frac{v}{u}$

$v = - 3 u$

$| v |$ $-$ $| u |$ $= 40$

$u = 20 cm$

$v = 60 cm$

$∴$ $\frac{1}{v} + \frac{1}{u} = \frac{1}{f}$

$\frac{1}{- 60} + \frac{1}{- 20} = \frac{1}{f}$

$f = - 15 cm$

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