In the case of diffraction bands due to a straight edge, as we move away from the edge

Options

A. intensity of the bright band increases and that of the dark band decreases
B. intensity of the bright band decreases and that of the dark band increases
C. intensity of the bright band decreases but that of the dark band remains unchanged
D. intensity of the bright band remains unchanged but that of the dark band increases

Answer

To solve the question regarding the behavior of diffraction bands due to a straight edge as we move away from the edge, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Diffraction**: Diffraction occurs when waves encounter an obstacle or a slit that is comparable in size to their wavelength. In this case, we are considering a straight edge.

2. **Identify the Bands**: In the diffraction pattern created by a straight edge, we observe alternating bright and dark bands. The bright bands are areas of constructive interference, while the dark bands are areas of destructive interference.

3. **Analyze the Intensity of Bright Bands**: As we move away from the edge, the intensity of the bright bands tends to decrease. This is because the waves spreading out from the edge lose coherence and the amplitude of the waves decreases with distance.

4. **Analyze the Intensity of Dark Bands**: The intensity of the dark bands, which corresponds to areas of destructive interference, remains relatively unchanged as we move away from the edge. This is because the conditions for destructive interference are still met, even at greater distances.

5. **Evaluate the Options**: Based on the analysis:
- Option 1: Bright band decreases, dark band decreases - Incorrect.
- Option 2: Bright band decreases, dark band increases - Incorrect.
- Option 3: Bright band decreases, dark band unchanged - Correct.
- Option 4: Bright band unchanged, dark band increases - Incorrect.

6. **Conclusion**: The correct statement is that the intensity of the bright band decreases while the intensity of the dark band remains unchanged as we move away from the edge.

### Final Answer:
The correct option is: **The intensity of the bright band decreases and the dark band remains unchanged.**

---