A convex lens is placed in contact with a plane mirror. An axial point object at a distance of `20 cm` from this combination, has its image coinciding with itself. What is the focal length of the convex lens ?

Explanation

To solve the problem, we need to analyze the situation involving a convex lens placed in contact with a plane mirror and an axial point object. Here's a step-by-step solution:

### Step 1: Understand the Setup
We have a convex lens and a plane mirror placed in contact. An object is located 20 cm away from this combination. The image formed by the lens and mirror coincides with the object itself.

### Step 2: Identify the Object Distance
The distance of the object from the lens-mirror combination is given as 20 cm. We denote this distance as \( u = -20 \, \text{cm} \) (the negative sign indicates that the object is on the same side as the incoming light).

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