Why are potassium and caesium, rather lithium used in photoelectric cells?

Explanation

To understand why potassium and cesium are preferred over lithium in photoelectric cells, we need to consider the properties of these alkali metals, particularly their ionization enthalpy and atomic size.

### Step-by-Step Solution:

1. **Understanding Photoelectric Effect**:
- The photoelectric effect occurs when light strikes a material and causes it to emit electrons. For this to happen, the energy of the incoming light must be sufficient to overcome the energy holding the electrons in the material.

2. **Ionization Enthalpy**:
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