In the figure shown here, what is the equivalent focal length of the combination of lenses (Assume that all layers are thin)?

Step-by-step solution

The focal length f L of the left plano-concave lens can be calculated as follows- 1 f L = n - 1 1 R 1 - 1 R 2 =   1 . 6 - 1 1 ∞ - 1 20   cm =   - 0 . 6 20   cm       . . . 1 The focal length f M of the middle bi-convex lens can be calculated as follows- 1 f M = n - 1 1 R 1 - 1 R 2 =   1 . 5 - 1 1 20   cm - 1 - 20   cm =   0 . 5 10   cm       . . . 2 The focal length f R of the right plano-concave lens can be calculated as follows- 1 f