A ray of light is travelling at an angle of 20° above the horizontal. At what angle with the horizontal must a plane mirror be placed in its path so that it becomes vertically upward after reflection.

Explanation

To solve the problem of determining the angle at which a plane mirror must be placed so that a ray of light traveling at an angle of 20° above the horizontal reflects vertically upward, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Incident Ray**:
- The ray of light is traveling at an angle of 20° above the horizontal. This means that the angle of the incident ray with respect to the horizontal line is 20°.

2. **Setting Up the Mirror**:
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