A discharge tube appears dark when evacuated to extremely low pressure . Why ?

Explanation

### Step-by-Step Solution:

1. **Understanding the Discharge Tube**:
- A discharge tube is a sealed glass tube from which most of the air has been evacuated. It contains two electrodes: a cathode (negative) and an anode (positive). When a high voltage is applied, electrons are emitted from the cathode and move towards the anode, creating cathode rays.

2. **Evacuation of the Tube**:
- When the discharge tube is evacuated to extremely low pressure, the number of gas molecules inside the tube is significantly reduced. This creates a vacuum environment.

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