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, we need to determine the angle at which the plane mirror should be placed so that a ray of light traveling at an angle of 20° above the horizontal reflects and travels vertically upward (90° with the horizontal).

### 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 incidence (i) with respect to the normal (which is perpendicular to the mirror surface) will be 70° (since the normal makes a 90° angle with the horizontal).

2. **Law of Reflection**:
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