Two thin lenses of power `+ 6 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:**
- Let the power of the first lens, \( P_1 = +6D \).
- Let the power of the second lens, \( P_2 = -2D \).

...