Question

The power of a lens is -4 D. Find the focal length and type of the lens?

• A. 25 cm, convex lens
• B. 25 cm, concave lens
• C. 40 cm, concave lens
• D. 40 cm, convex lens

Answer 1

The correct option is B

25 cm, concave lens

Given, Power of the lens $P=-4D$

We know, $P=\frac{1}{focallength}$

So, $focallength=\frac{1}{-4}=-0.25m=-25cm$.

Minus sign indicates that it is a concave lens, as the focal length of a concave lens is negative.

Therefore, it is a concave lens of focal length 25 cm.