A : In Young.s double slit experiment, the fringes become indistinct if one of the slits is covered with cellophane paper.
R : The cellophane paper decrease the wavelength of light.

Options

A. Both A and R are true and R is the correct explanation of A
B. Both A and R are true and R is not the correct explanation of A
C. A is true and R is false
D. Both A and R are false

Answer

To analyze the given statements in the context of Young's double slit experiment, we can break down the solution into the following steps:

### Step 1: Understanding the Assertion
The assertion states that in Young's double slit experiment, the fringes become indistinct if one of the slits is covered with cellophane paper.

- **Explanation**: In Young's double slit experiment, distinct fringes are formed due to the interference of light waves coming from the two slits. If one slit is covered with cellophane paper, the light passing through it will undergo some absorption and scattering, which can affect the clarity of the interference pattern.

### Step 2: Analyzing the Effect of Cellophane Paper
When one slit is covered with cellophane paper, it acts as a translucent medium.

- **Explanation**: A translucent medium allows some light to pass through but also absorbs some of it. This means that the intensity of light coming from the slit covered with cellophane will be less than that from the other slit.

### Step 3: Impact on Interference Pattern
The interference pattern is dependent on the intensity of light from both slits.

- **Explanation**: For distinct fringes to be observed, the intensities of light from both slits must be equal (I1 = I2). When one slit has reduced intensity due to the cellophane paper, the contrast between the bright and dark fringes decreases, making them indistinct.

### Step 4: Understanding the Reason
The reason states that the cellophane paper decreases the wavelength of light.

- **Explanation**: This statement is incorrect. The wavelength of light does not change when it passes through a medium like cellophane paper. While the speed of light and its intensity may change, the wavelength of light remains the same when it exits the medium.

### Step 5: Conclusion
Based on the analysis:

- The assertion is **true**: The fringes become indistinct when one slit is covered with cellophane paper.
- The reason is **false**: The cellophane paper does not decrease the wavelength of light.

Thus, the final conclusion is that the assertion is true, but the reason is false.

### Final Answer
Assertion (A) is true, and Reason (R) is false.

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