Explain thermionic emission, field emission and photoelectric emission.

Explanation

### Step-by-Step Solution:

1. **Thermionic Emission:**
- Thermionic emission refers to the process where electrons are emitted from a material (usually a metal) when it is heated to a high temperature.
- As the temperature increases, the thermal energy provided to the electrons in the material becomes sufficient to overcome the work function (the minimum energy needed to remove an electron from the surface of the material).
- When the electrons gain enough energy, they can escape from the material into the surrounding space. This phenomenon is commonly observed in vacuum tubes and cathode ray tubes.

2. **Field Emission:**
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