How do you measure quickly the approximately focal length of a concave mirror?

Explanation

To measure the approximate focal length of a concave mirror quickly, follow these steps:

### Step-by-Step Solution:

1. **Gather Materials**: You will need a concave mirror, a cardboard screen (or any flat surface), and a distant object (like a tree or a building).

2. **Position the Mirror**: Hold the concave mirror facing a distant object. Ensure that the object is far enough away (ideally at infinity) so that the light rays coming from it are parallel.

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