What is the focal length of common looking glass?

Explanation

To determine the focal length of a common looking glass (which is typically a concave mirror), we need to understand a few key concepts about lenses and mirrors.

### Step-by-Step Solution:

1. **Understanding Focal Length**:
- The focal length (f) of a lens or mirror is defined as the distance from the optical center (O) to the focus (F). The focus is the point where parallel rays of light either converge (for converging lenses/mirrors) or appear to diverge from (for diverging lenses/mirrors).

2. **Relation to Radius of Curvature**:
...