Question

A convergent beam seems to converge at a point which is $15cm$ behind a mirror, but due to reflection at the mirror, the beam diverges and seems to emerge from a point which is $12cm$ behind mirror, find focal length of mirror.

• A. $f=-20/3cm$
• B. $f=+10/3cm$
• C. $f=+20/3cm$
• D. $f=+10cm$

Answer 1

The correct option is C $f=+20/3cm$

$u=+15cm$
$v=+12cm$
Applying mirror formula,
$\frac{1}{v}+\frac{1}{u}=\frac{1}{f}$

$\frac{1}{15}+\frac{1}{12}=\frac{1}{f}$

$\frac{4}{60}+\frac{5}{60}=\frac{1}{f}$

$f=\frac{60}{9}=\frac{20}{3}=\frac{+20}{3}cm$