Question

when an object placed 20 cm from a concave mirror areal image magnified three times is formed find the focal length of mirror​

Answer 1

$\red{\large {\underline{ \underline{ \star \: {Given}}}}}$

U = -20 Cm.

M = -3.

$\red{\large {\underline{ \underline{ \star \: {Solution}}}}}$

$m = \frac{ - v}{u} . \\ \\ \implies\frac{ \cancel - 3}{1} = \frac{ \cancel - v}{u} . \\ \\ \implies 3u = v. - - - - (1)$

Give the value of u = -20.

By equation (1)

3(-20) = v

-60 = v.

Now,

Using Mirror Formula.

$\frac{1}{f} = \frac{1}{v} + \frac{1}{u} . \\ \\ \implies \frac{1}{f} = \frac{1}{ - 60} + \frac{1}{ - 20} . \\ \\ \implies \frac{1}{f} = \frac{ - 1 - 3}{60} . \\ \\ \implies \frac{1}{f} = \frac{ - 4}{60} . \\ \\ \implies f = - \frac{\cancel6\cancel0}{\cancel4}\\ \\\implies f=-15.$