Question

A dentist uses a small concave mirror of focal length 3 cm and hold it at a distance of 2 cm from the tooth. Which of the following statement is incorrect about the image formed?

• A. The image is three times the size of the object.
• B. The image distance is -6 cm from the pole.
• C. The image formed is erect.
• D. The distance between the object and the image is 8 cm.

Answer 1

The correct option is B The image distance is -6 cm from the pole.

Given :
Focal length, $f=-3cm$
Object distance, $u=-2cm$

Using mirror formula,
$\frac{1}{\text{f}}=\frac{1}{\text{v}}+\frac{1}{\text{u}}$
$\frac{1}{\text{-3}}=\frac{1}{\text{v}}+\frac{1}{\text{-2}}$
$v=6cm$

$Magnification=\frac{h_i}{h_o}-\frac{v}{u}=-\frac{6}{-2}=3$

Image formed is three times of the object size and it is virtual and erect.