`K` and `Cs` are used in photoelectric cells.
`K` and `Cs` emit electrons on exposure to light.

Options

A. Both A and R true and R is the correct explanation of A
B. Both A and R are true but R is not a correct explanation of A
C. A is true but R is false
D. A is false but R is true

Answer

To solve the question regarding the use of Potassium (K) and Cesium (Cs) in photoelectric cells, we will break it down step by step.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that Potassium (K) and Cesium (Cs) are used in photoelectric cells.
- Both of these elements are alkali metals.

2. **Properties of Alkali Metals**:
- Alkali metals have a general electronic configuration of \( ns^1 \), meaning they have one electron in their outermost shell.
- This single valence electron is loosely bound to the nucleus, making it easier for these metals to lose this electron.

3. **Photoelectric Effect**:
- The photoelectric effect refers to the phenomenon where electrons are emitted from a material (in this case, K or Cs) when it is exposed to light of sufficient energy.
- When light hits the surface of the metal, the energy from the light is absorbed by the electron, which can then overcome the attractive forces holding it to the metal and be emitted.

4. **Reasoning**:
- The reason provided states that K and Cs emit electrons upon exposure to light, which is true. This is due to their low ionization energy and the presence of a single valence electron.
- Thus, the reason supports the assertion that these metals are used in photoelectric cells.

5. **Conclusion**:
- Both the assertion (that K and Cs are used in photoelectric cells) and the reason (that they emit electrons on exposure to light) are true.
- Therefore, the correct answer is that both the assertion and reason are true, and the reason is the correct explanation for the assertion.

### Final Answer:
Both assertion and reason are true, and the reason is the correct explanation for the assertion.

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