Question

If the radii of curvature of the faces of a double convex lens are 10 cm and 20 cm. Its focal
length is 15 cm. What is the refractive index of the material of the lens?

Answer 1

Answer:

Here you just need the lens makers formula in air:

1/f = (n-1)(1/R1 - 1/R2), where R2 > R1.

In this problem, R2 is going to be 20 cm and R1 = 10 cm with f = 10 cm. You just need to solve for n.

EDIT, [correction]: I just realized that R1 and R2 are the same sign only when the radii are measured from the same side. In this case, R2 should be negative which cancels out the minus so that its actually ( 1/R1 + 1/R2) if positive values are used.

Explanation:

hope it's useful to u