Assertion if wavelength is of the order of distance between the slits, then figure size is large.
Reason Fringe width is given by `beta=lambdaD//d`

Answer

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

### Step 1: Understand the Assertion
The assertion states that "if the wavelength is of the order of the distance between the slits, then the figure size is large." This implies that when the wavelength (λ) is comparable to the distance between the slits (d), the resulting interference pattern on the screen will be more pronounced and larger in size.

**Hint:** Consider how the relationship between wavelength and slit distance affects the interference pattern.

### Step 2: Understand the Reason
The reason given is that "fringe width (β) is given by the formula β = λD/d," where:
- β is the fringe width,
- λ is the wavelength,
- D is the distance from the slits to the screen,
- d is the distance between the slits.

This formula indicates that the fringe width is directly proportional to the wavelength and the distance to the screen, while it is inversely proportional to the distance between the slits.

**Hint:** Analyze how changing the value of d affects the fringe width.

### Step 3: Analyze the Relationship
From the formula β = λD/d, we can see that:
- If d (the distance between the slits) decreases, the fringe width β increases.
- A larger fringe width means that the interference pattern will spread out more, leading to a larger figure size on the screen.

**Hint:** Think about what happens to the interference pattern as the distance between the slits decreases.

### Step 4: Conclusion
Since both the assertion and the reason are true, and the reason correctly explains the assertion, we conclude that:
- The assertion is true: "if the wavelength is of the order of distance between the slits, then figure size is large."
- The reason is also true: "fringe width is given by β = λD/d" and explains why the assertion holds.

Thus, both the assertion and reason are correct, and the reason is the correct explanation of the assertion.

**Final Answer:** Both assertion and reason are correct, and the reason is the correct explanation of the assertion.