Question

a) What are the various differences between a real image and a virtual image?
b) 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

Part a): 2 Marks
part b): 3 Marks


The various differences between a real image and a virtual image are as followed:
(i) A real image is formed when the actual intersection of light rays happens. In virtual images, there is no intersection of light rays.
(ii) Real images are always inverted. Virtual images are erect.

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.