Question

A convex lens of power +5 D has an object placed at a distance of 50 cm in front of the lens.

1. State the nature of the image formed.
2. What is the definite distance, towards or away from the lens, the object should move so that the image is not seen?

Answer 1

Focal length \[ f = \frac{1}{\mathrm{P}} = \frac{1}{5} \ \text{m} = 0.2 \ \text{m} = 20 \ \text{cm} \]

1. The object is 50 cm from the lens, which is beyond 2f = 40 cm. So the image is real, inverted and diminished, formed between (F) and (2F) on the other side of the lens.
2. A real image is not formed on a screen when the object is at the principal focus; the image is then at infinity. The object must move from 50 cm to 20 cm from the lens, so it should move 30 cm towards the lens.