Question

The power of the combination of two lenses is −5 D. If one is a convex lens with a focal length of 20 cm, then what is the power of the other lens?​

Answer 1

Answer:

Assuming the two lenses are in contact we have 1/f = 1/f1 + 1/f2  

where the total power 1/f = power = 5D  

We know, P = 1 / f (that is, 1D) where f = 1 m. The units of dioptre is 1/m .

Therefore, 100/15 + 1/f2 = 5  

1/f2 = -25/15  

f2 = -15/25  

= -0.6 = -60 cm.

Hence, the focal length of B is -60 cm

Explanation: