Assertion: In the figure shown, zero order maxima will lie above point O.
Reason: Zero order maxima normally means a point where path difference is zero.

Options

A. (a)If both Assertion and Reason are true and the Reason is correct explanation of the Assertion.
B. (b)If both Assertion and Reason are true and the Reason is not the correct explanation of the Assertion.
C. (c) If Assertion is true, but the Reason is false.
D. (d) If Assertion is false, but the Reason is true.

Answer

To solve the question, we need to analyze the assertion and reason provided in the context of the Young's double slit experiment.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
The assertion states that the zero order maxima will lie above point O. In the context of the Young's double slit experiment, the zero order maxima refers to the point where the path difference between the two waves from the slits is zero.

**Hint**: Recall the definition of zero order maxima in interference patterns.

2. **Understanding the Reason**:
The reason given is that zero order maxima normally means a point where the path difference is zero. This is indeed true, as the zero order maxima occurs at the center of the interference pattern where the waves from both slits arrive in phase.

**Hint**: Think about how path difference affects the interference pattern.

3. **Analyzing the Setup**:
In the given scenario, there is a slab of refractive index \( \mu \) placed in front of one of the sources. This slab will change the effective path length of the light coming from that source. The path difference introduced by the slab is given by \( (\mu - 1) \cdot t \), where \( t \) is the thickness of the slab.

**Hint**: Consider how the introduction of a medium affects the speed of light and consequently the path length.

4. **Determining the Effect on Maxima Position**:
Since the path length from the source behind the slab increases due to the refractive index, to achieve a zero path difference, we need to adjust our observation point. The zero order maxima will shift upward from point O because we need to compensate for the additional path length introduced by the slab.

**Hint**: Visualize how the path difference changes the position of the maxima.

5. **Conclusion**:
Both the assertion and reason are true. However, the reason does not correctly explain the assertion because the assertion's correctness relies on the understanding of how the slab changes the path difference, not merely the definition of zero order maxima.

**Final Answer**: The assertion is true, the reason is true, but the reason is not the correct explanation of the assertion.

### Summary of the Answer:
- Assertion: True
- Reason: True
- Explanation: The reason does not correctly explain the assertion.

### Final Answer Option: B