Question

A 2 cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 10 cm. The distance of the object from the lens is 15 cm. Find the position and nature of the image. [3 Marks]

Answer 1

Given
Object distance, $u=-15cm$
Focal length, $f=+10cm$
Image distance, $v=?$ (1 Mark)

Using the lens formula,
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$ (1 Mark)

$\frac{1}{v}-\frac{1}{-15}=\frac{1}{+10}$
$\frac{1}{v}=\frac{1}{10}-\frac{1}{15}$
$\frac{1}{v}=\frac{3-2}{30}$
$\frac{1}{v}=\frac{1}{30}$
$v=30cm$

Positive sign of $v$ shows that the image is formed at a distance of 30 cm on the right side of the lens. Therefore, the image is real and inverted. (1 Mark)