In a `p-n` junction photo cell, the value of the photo electromotive force produced by monochromatic light is proportional to

Options

A. a. The intensity of the light falling on the cell
B. b. The frequency of the light falling on the cell
C. c. The voltage applied at the p–n junction
D. d. The barrier voltage at the p–n junction

Explanation

To solve the question regarding the photo electromotive force (emf) produced by a p-n junction photo cell, we will analyze the relationship between the emf and the factors involved when monochromatic light falls on the junction.

### Step-by-Step Solution:

1. **Understanding the p-n Junction**:
- A p-n junction is formed by joining p-type and n-type semiconductors. When light falls on this junction, it can generate electron-hole pairs.

2. **Effect of Light on the Junction**:
- When monochromatic light (light of a single wavelength) strikes the p-n junction, photons are absorbed by the semiconductor material. Each photon can excite an electron from the valence band to the conduction band, creating an electron-hole pair.

3. **Proportionality to the Number of Photons**:
- The number of electron-hole pairs generated is directly proportional to the number of photons hitting the junction. This means that if more light (photons) falls on the junction, more electron-hole pairs will be created.

4. **Intensity of Light**:
- The intensity of light is defined as the power per unit area. It is related to the number of photons incident on the surface per unit time. Therefore, as the intensity of the light increases, the number of photons increases, leading to more electron-hole pairs being generated.

5. **Relationship of emf to Intensity**:
- The photo electromotive force (emf) generated in the p-n junction is proportional to the number of electron-hole pairs created. Since the number of electron-hole pairs is proportional to the intensity of the light, we can conclude that the emf produced is also proportional to the intensity of the light falling on the junction.

6. **Conclusion**:
- Therefore, the correct answer to the question is that the photo electromotive force produced by monochromatic light in a p-n junction photo cell is proportional to the intensity of light falling on the cell.

### Final Answer:
The photo electromotive force produced by monochromatic light in a p-n junction photo cell is proportional to the **intensity of light falling on the cell**.