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 1: Identify the Powers of the Lenses
We have two lenses:
- Lens 1 has a power \( P_1 = +4 \, \text{D} \)
- Lens 2 has a power \( P_2 = -2 \, \text{D} \)

### Step 2: Calculate the Total Power of the Combination
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