Question

A convex lens of focal length 30 cm is placed coaxially in contact with a concave lens of focal length 40 cm. Determine the power of the combination. Will the system be converging or diverging in nature?

Answer 1

We have,focal lengh of convex lens,f₁= +30 cm = +0.30 m and focal length 

of concave lens, f2 = - 40 cm = - 0.40m

Equivalent focal lenght, `f = 1/f^1  + 1/f^2  = 1/30  + 1/-40  = (40 - 30)/1200  = 1/120 `

∴ F = 120 cm = 1.2 m

∴ Power of the combination , `p=1/F = 1/1.2 = 0.83 D`

The focal length of the combination = 1.2 m = 120 cm

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