Question

what will be the power of compound lens made by combing convex and concave lens of equal power?

Answer 1

Since the lens addition formula adds the reciprocal of the component focal lengths , the resulting focal length will be 1/2 the original focal length.

Here’s the break down: 1/f = 1/f1 + 1/f2. where f1 = f2.

1/f = 1/f1 + 1/f1 = 2/f1, so f = f1/2.

Similarly, if three lenses were placed in contact, then the combined focal length would be 1/3 of a single lens.