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**:
- The first lens has a power of \( P_1 = +6 \, \text{D} \) (convex lens).
- The second lens has a power of \( P_2 = -2 \, \text{D} \) (concave lens).

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