Two thin lenses of power +5 D and -2.5 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 are given two lenses:
- Lens 1: Power \( P_1 = +5 \, D \)
- Lens 2: Power \( P_2 = -2.5 \, D \)

### Step 2: Calculate the total power of the combination
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