Assertion `:` There is no dispersion of light refracted through a rectangular glass slab.
Reason `:` Dispersion of light is the phenomenon of splitting of a beam of white light into its constituent colours.

Options

A. If both Assertion and Reason are true and Reason is the correct explanation of Assertion.
B. If both Assertion and Reason are true but Reason is not correct explanation of Assertion.
C. If Assertion is true but Reason is false.
D. If Assertion is false but Reason is true.

Explanation

To solve the assertion and reason question, we need to analyze both statements and understand their implications regarding the phenomenon of light refraction and dispersion.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "there is no dispersion of light refracted through a rectangular glass slab."
- To understand this, we need to consider how light behaves when it passes through a rectangular glass slab.

2. **Drawing the Diagram**:
- Imagine a rectangular glass slab with two parallel surfaces.
- When a beam of white light (which consists of multiple colors) enters the slab, it refracts at the first surface and again at the second surface.

3. **Analyzing Refraction**:
- When light enters the glass slab, it bends towards the normal due to a change in medium (from air to glass).
- Upon exiting the slab, it bends away from the normal.
- Importantly, because the surfaces of the slab are parallel, the angle of incidence and the angle of refraction at the second surface will ensure that the emergent ray is parallel to the incident ray.

4. **Conclusion on Dispersion**:
- Since the incident and emergent rays are parallel, all colors of light (which would normally disperse at different angles) exit the slab in the same direction.
- Therefore, there is no dispersion of light when it passes through a rectangular glass slab, confirming that the assertion is true.

5. **Understanding the Reason**:
- The reason states that "dispersion of light is the phenomenon of splitting of a beam of white light into its constituent colors."
- This is indeed a correct definition of dispersion, where white light separates into its component colors (like a rainbow).

6. **Evaluating the Relationship**:
- While both the assertion and the reason are true, the reason does not explain why there is no dispersion in the case of a rectangular glass slab.
- The assertion is specific to the behavior of light in a rectangular slab, while the reason is a general definition of dispersion.

7. **Final Conclusion**:
- Therefore, the correct assessment is that both the assertion and reason are true, but the reason is not the correct explanation for the assertion.

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