Define the terms focus and focal length of a concave mirror. Draw diagram to illustrate your answer.

Explanation

### Step-by-Step Solution:

1. **Define Focus:**
- The focus of a concave mirror is defined as the point on the principal axis where all parallel rays of light converge after reflecting off the mirror. It is denoted by the letter 'F'.

2. **Explain the Principle:**
- When parallel rays of light strike a concave mirror, they reflect off the surface and converge at the focus. This is a key characteristic of concave mirrors, which makes them useful in applications like telescopes and shaving mirrors.

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