Question

a concave lens has focal length of 15 cm. at what distance should an object be placed from the lens so that if forms an image at 10cm from the lens? Find the magnification of the lens. (-30cm,1/3)

Answer 1

Given :-

$\sf {f = - 15\ cm}\\\\\\\\sf {v = - 10\ cm}$

To Find :-

u

m

Solution :-

$\sf {Using\ \frac{1}{v} - \frac{1}{u} =\frac{1}{f}$

$\sf {\mapsto \frac{1}{-10}- \frac{1}{yu}= \frac{1}{-15} }\\\\\\\sf {\mapsto \frac{1}{u}= \frac{-1}{10}+ \frac{1}{15} }\\\\\\\sf {\mapsto \frac{1}{u}= -3 + \frac{2}{30} }\\\\\\\sf {\mapsto \frac{1}{u}= \frac{1}{30}}\\\\\\\sf {\mapsto u=-30\ cm }\\\\\\\\\sf {Magnification, m = \frac{v}{u} }\\\\\\\sf {\mapsto m=\frac{-10}{-30}}\\\\\\\sf {\mapsto m= \frac{1}{3} }$