Question

The radii of curvature of both the sides of a convex lens are 15 cm and if the refractive index of the material of the lens is 1.5, then focal length of lens in air is _______ cm.
(A) 10
(B) 15
(C) 20
(D) 30

Answer 1

In case you need to be correct, this relies upon the thickness D of the focal point.

In case you expect the thickness to be zero, you can simply add the central forces to get the aggregate central power: 1/f12=p12 = p1+p2-p1*p2*D12/n = ~ p1+p2 With pi = (nRi-nLi)/Ri

So for this situation the main surface has p1=(1.5– 1)/(30cm) = 1/(60cm), the second surface p2=(1– 1.5)/(- 15cm)=1/(30cm), so the option is p12 = 3/(60cm), so add up to central length f12 = 60/3cm = 20cm.