Question

4. A lens of power -4D. Calculate the focal length.​

Answer 1

Answer:

Hence, focal length = -0. 25 m = -25 cm

Explanation:

We know that,

Power, P = 1/focal length (in metres), f

=> P = 1 / f

Here, given,

Power = -4D

Then, by applying values, we get,

-4 = 1 / f

=> f = 1/(-4)

=> f = -0. 25 m

This shows the given lens is concave which produces virtual and erect image.

Hence, focal length = -0. 25 m = -25 cm

Answer 2

Answer:

Given,

Power= -4D

Explanation:we the lense formula

Power=1/focal length

So, As we given

-4D=1/f

or, f= 1/ -4

Or, f= -0.25

So, the focal length is 0.25m or 25cm