Two concave mirrors of equal focal lengths are placed facing each other with their principal axes coinciding. An object O is to be placed at the mid-point between mirrors. What should be the separation between mirrors so that only one image is formed?

Explanation

To solve the problem, we need to determine the separation between two concave mirrors so that only one image is formed when an object is placed at the midpoint between them. Here's a step-by-step solution:

### Step-by-Step Solution

1. **Understanding the Setup**:
- We have two concave mirrors facing each other with their principal axes aligned.
- The focal lengths of both mirrors are equal, denoted as \( f \).
- The object \( O \) is placed at the midpoint between the two mirrors.
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