Assertion:- In standard YDSE set up with visible light, the position on screen where phase difference is zero appears bright.
Reason:- In YDSE set up magnitude of electromagnetic field at central bright frings is not varying with time.

Options

A. If both Assertion `&` Reason are True `&` the Reason is a correct explanation of the Assertion.
B. If both Assertion `&` Reason are True but Reason is not a correct explanation of the Assertion.
C. If Assertion is True but the Reason is False.
D. If both Assertion `&` Reason are False.

Explanation

To solve the given question, we need to analyze both the assertion and the reason provided in the context of Young's Double Slit Experiment (YDSE).

### Step-by-Step Solution

1. **Understanding the Assertion**:
- The assertion states that in a standard YDSE setup with visible light, the position on the screen where the phase difference is zero appears bright.
- In YDSE, constructive interference occurs when the path difference between the two waves arriving at a point on the screen is an integer multiple of the wavelength (nλ, where n = 0, 1, 2,...). At the center of the screen (where the path difference is zero), the phase difference is also zero, leading to maximum intensity and a bright fringe.

**Hint**: Recall the condition for constructive interference in YDSE.

2. **Understanding the Reason**:
- The reason states that in the YDSE setup, the magnitude of the electromagnetic field at the central bright fringe is not varying with time.
- The electric and magnetic fields of the light waves at the central bright fringe are indeed in phase and add constructively. The resultant electric field remains constant in magnitude at this point, leading to a stable intensity.

**Hint**: Consider how the electric and magnetic fields behave at points of constructive interference.

3. **Evaluating the Truth of the Statements**:
- Both the assertion and the reason are true statements. The assertion correctly describes the condition for a bright fringe, and the reason correctly describes the behavior of the electromagnetic field at the central bright fringe.
- However, the reason does not directly explain why the assertion is true. The assertion is about the appearance of the bright fringe due to phase difference, while the reason discusses the stability of the electromagnetic field.

**Hint**: Analyze whether the reason logically supports the assertion.

4. **Conclusion**:
- Since both the assertion and the reason are true, but the reason does not provide a correct explanation for the assertion, the correct answer is that both statements are true, but the reason is not the correct explanation for the assertion.

### Final Answer:
Both the assertion and the reason are true, but the reason is not the correct explanation for the assertion.