A: When a narrow pulse of light is sent through a medium, it doesn't retain its shape as it travels through the medium.
R: Since the speed of propagation in a medium depends on wavelength , different wavelength components of the pulse travel with different speeds.

Options

A. IF both Assertion & Reason are true and the reason is the correct explanation of the assertion, then mark (1).
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion, then mark(2)
C. IF Assertion is true statement but Reason is false then mark(3)
D. If both Assertion and Reason are false statements, then mark (4)

Answer

To solve the question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that when a narrow pulse of light is sent through a medium, it does not retain its shape as it travels through the medium. This implies that the pulse spreads out or changes shape while propagating.

2. **Understanding the Reason**:
- The reason given is that the speed of propagation in a medium depends on the wavelength. Different wavelength components of the pulse travel with different speeds. This means that if the pulse consists of multiple wavelengths, each wavelength will travel at a different speed in the medium.

3. **Analyzing the Effect of Wavelength on Speed**:
- The speed of light in a medium can be expressed as \( V = f \times \lambda \), where \( V \) is the speed, \( f \) is the frequency, and \( \lambda \) is the wavelength.
- When light enters a medium from air, the wavelength changes due to the refractive index of the medium, but the frequency remains constant. Therefore, if the wavelength changes, the speed of light in the medium will also change.

4. **Implication of Different Speeds**:
- Since different wavelengths travel at different speeds, the components of the pulse will separate as they propagate through the medium. This leads to the distortion of the pulse shape, confirming the assertion.

5. **Conclusion**:
- Both the assertion and the reason are true. The reason correctly explains why the assertion is true. Therefore, the correct answer is that both the assertion and reason are true, and the reason is the correct explanation for the assertion.

### Final Answer:
Both assertion and reason are true, and the reason is the correct explanation for the assertion.

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