Assertion : A beam of the white light shows no dispersion on emerging from a glass slab.
Reason : Dispersion in a glass slab is zero.

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 and reason is not the correct explanation of assertion.
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Explanation

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

### Step 1: Understand the Assertion
The assertion states that "A beam of white light shows no dispersion on emerging from a glass slab."

**Explanation**: When white light enters a glass slab, it is made up of different colors (red, orange, yellow, green, blue, indigo, violet). Each of these colors has a different wavelength and refracts at slightly different angles due to the varying speeds of light in the glass. However, when the light exits the slab, all the colors emerge together as white light because they are parallel to each other. Thus, there is no visible dispersion when the light exits the slab.

### Step 2: Understand the Reason
The reason states that "Dispersion in a glass slab is zero."

**Explanation**: Dispersion refers to the separation of light into its constituent colors. Inside the glass slab, there is indeed dispersion as the different colors refract at different angles. However, because the glass slab has parallel sides, the light rays that are dispersed inside the slab are brought back together when they exit. Therefore, while there is dispersion occurring inside the slab, it does not result in a visible separation of colors when the light emerges.

### Step 3: Analyze the Relationship
Now we need to determine if the reason correctly explains the assertion.

- The assertion is true: White light does not show dispersion upon emerging from the glass slab.
- The reason is also true: There is dispersion occurring within the glass slab. However, it does not explain why the assertion is true, as the reason incorrectly states that dispersion is zero.

### Conclusion
Both the assertion and reason are true, but the reason does not provide a correct explanation for the assertion. Therefore, the correct answer is:

**Both assertion and reason are true, but reason is not the correct explanation of assertion.**

### Final Answer
**Option B**: Both assertion and reason are true, but reason is not the correct explanation of assertion.

---