Two thin lenses of power +4 D and -2 D are in contact. What is the focal length of the combination ?

Explanation

To find the focal length of the combination of two thin lenses in contact, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Powers of the Lenses:**
- The power of the first lens (P1) is +4 D (diopters).
- The power of the second lens (P2) is -2 D.

...