Question

A doctor looks through a small hole at the vertex of a concave mirror to examine a sore throat. If the radius of curvature of the mirror is $40cm$ and the light source is $1m$ from the mirror, the distance from the throat should the mirror be if the light is to be imaged on the inflamed area is:

• A. $-50cm$
• B. $-25cm$
• C. $40cm$
• D. $20cm$

Answer 1

Answer: (B)

$-25cm$

$f=-\frac{R}{2}=-\frac{40}{2}=-20cm$
$u=-100cm$
From mirror equation:
$\frac{1}{f}=\frac{1}{u}+\frac{1}{v}$
$\Rightarrow \frac{1}{-20}=\frac{1}{-100}+\frac{1}{v}$
$\Rightarrow \frac{1}{v}=\frac{1}{-20}-\frac{1}{-100}\Rightarrow v=-25cm$