A concave mirror forms a virtual image of size twice that of the object placed at a distance 5 cm from it . Find the focal length of the mirror.

Explanation

To solve the problem step-by-step, we will follow the concepts of optics related to concave mirrors and magnification.

### Step 1: Identify the given values
- The object distance (u) is given as 5 cm. According to the sign convention, for mirrors, the object distance is taken as negative. Therefore, \( u = -5 \, \text{cm} \).
- The image size is twice the object size, which means the magnification (m) is 2. Since the image is virtual and erect, the magnification is positive: \( m = +2 \).

### Step 2: Use the magnification formula
The magnification (m) for mirrors is given by the formula:
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