Question

A convex lens of focal length f₁ is kept in contact with a concave lens of focal length f₂. Find the focal length of the combination.

Answer 1

For convex lens of focal length (+ f1) + 1/f1 = 1/v' - 1/u (1) For concave lens of focal length (– f2) - 1/f2 = 1/v - 1/v' (2) Adding equation (1) and (2) 1/f1 - 1/f2 = 1/v - 1/u (3) For an equivalent lens (using lens formula) 1/f = 1/v - 1/u,where f is the focal length of combination. (4) From equation (3) and (4), 1/f = 1/f1 - 1/f2

Answer 2

Answer:

Explanation:

The distance between optical centre and principal

focus is called focal length of a lens