In the Fraunhofer class of diffraction

Options

A. The source of light and the screen are al finile distance from diffracting apertura
B. The source and the screen are at infinite distances from the aperture
C. The source and the Screen are very close to each other
D. Both (1) & (3)

Answer

To solve the question regarding the Fraunhofer class of diffraction, we need to analyze the conditions under which this type of diffraction occurs.

### Step-by-Step Solution:

1. **Understanding Fraunhofer Diffraction**:
- Fraunhofer diffraction occurs when the light source and the observation screen (or detector) are both at a significant distance from the diffracting aperture. This allows the light rays to be considered as parallel.

2. **Identifying the Conditions**:
- In the context of the question, we have four options regarding the distances of the light source and the screen from the aperture:
- Option 1: The source and the screen are at finite distance from the diffracting aperture.
- Option 2: The source and the screen are at infinite distance from the aperture.
- Option 3: The source and the screen are very close to each other.
- Option 4: Both 1 and 3.

3. **Analyzing Each Option**:
- **Option 1**: If the source and screen are at finite distances, the rays will not be parallel, which does not satisfy the condition for Fraunhofer diffraction.
- **Option 2**: If both the source and screen are at infinite distances, the rays will be parallel, which is a requirement for Fraunhofer diffraction.
- **Option 3**: If the source and screen are very close to each other, the rays will also not be parallel, failing the condition for Fraunhofer diffraction.
- **Option 4**: This option suggests that both conditions from options 1 and 3 are valid, which is incorrect as neither satisfies the parallel ray condition.

4. **Conclusion**:
- The only option that satisfies the conditions for Fraunhofer diffraction is Option 2: "The source and the screen are at infinite distance from the aperture."

### Final Answer:
- The correct answer is **Option 2**: The source and the screen are at infinite distance from the aperture.