A thin convex lens of focal length 20 cm is kept in contact with a thin concave lens of focal length 15 cm. Find the focal length and the nature of the combination.

Explanation

To find the focal length and the nature of the combination of a thin convex lens and a thin concave lens in contact, we can follow these steps:

### Step 1: Identify the focal lengths
- The focal length of the convex lens (f1) is +20 cm (positive because it is a convex lens).
- The focal length of the concave lens (f2) is -15 cm (negative because it is a concave lens).

### Step 2: Use the lens formula for combination
The formula for the combined focal length (F) of two lenses in contact is given by:
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