Question

power of a lens is 0.4 D. what is its focal length?

Answer 1

Answer:

Explanation:

Power is the reciprocal of focal length in metres.

So,P=1/f

=> f=1/P=1/0.4=2.5m

Here, focal length is positive,so it is a convex lens.

If negative,it would be a concave lens.

Hope you understood

Answer 2

The power of the lens is calculated by using the formula:

$Power=\frac{1}{f} \\where, Power\ is\ in\ diopters\ and \ focal\ length\ is\ in\ meters.$

Given,

Power=$+0.4D$

Focal length=$\frac{1}{Power}$

Focal length=$\frac{1}{0.4} =2.5m\ or \ 250cm$

Hence, the lens' focal length with the power of +0.4D is 2.5m or250 cm.