Question

At what point a real image and virtual image are formed? A 3 cm high object is placed at a distance of 80 cm from a concave lens of focal length 20 cm. Find the position and size of the image. [5 MARKS]

Answer 1

Each point: 1 Mark
Problem: 3 Marks


1. A real image is formed at the point of intersection of (minimum 2) light rays
2. Virtual image is formed at the point from which light rays appear to diverge.

Problem:

Here, $u=-80cm,f=-20cm$

Using the len's formula, $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

$\frac{1}{v}-\frac{1}{-80}=\frac{1}{-20}$

$v=-16cm$

Magnification, $m=\frac{h^{{}^{\prime }}}{h}=\frac{v}{u}=\frac{-16}{-80}=\frac{1}{5}$

$h^{{}^{\prime }}=\frac{h}{5}=\frac{3.0}{5}=0.6cm$

Length of the image is 0.6 cm. Positive sign shows that the image is erect.