A concave lens is kept in contact with convex lens of focal length 20 cm. The combination works as a convex lens of focal length 50 cm. Find the power of concave lens.

Explanation

To solve the problem, we will follow these steps:

### Step 1: Understand the given information
We have a convex lens with a focal length \( F_1 = +20 \, \text{cm} \) (positive because it is a convex lens) and a combination of a concave lens and this convex lens that behaves like a convex lens with a focal length \( F = +50 \, \text{cm} \).

### Step 2: Use the lens formula for combined lenses
The formula for the focal length of two lenses in contact is given by:
\[
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