Question

A convex lens of focal length 20 cm is placed coaxially in contact with a concave lens of focal length 25 cm. Determine the power of the combination. Will the system be converging or diverging in nature?

Answer 1

We have

focal length of convex lens, f₁ = +20cm = +0.20 m

focal length of convex lens, f₂ = - 25cm = +0.25 m

Equivalent focal length, `F = 1/(f_1)+1/(f_1)=1/20+1/(-25)=1/(100)`

∴ F = 100cm

Power of convex lens, `P_1 = 1/f_1=1/0.20`

Power of concave lens, `P_2 = 1/f_2 = 1/(-0.25)`

Power of the combination, p = p₁ + p₂

`= 1/0.20 +1/-0.25`

`=100/20 +100/-25`

=`(500 - 400)/100`

=`100/100`

= 1 D

The focal length of the combination = 1 m = 100 cm

As the focal length is in positive, the system will be converging in nature.