A : Diffraction determines the limitations of the concept of light rays.
R : A beam of width `alpha` starts to spread out due to diffraction after it has travelled a diatance `(2alpha^(@)"/"lambda)`.

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 solve the question, we need to analyze the assertion (A) and the reason (R) given in the problem statement.

### Step 1: Understanding the Assertion (A)
The assertion states that "Diffraction determines the limitations of the concept of light rays."

- **Explanation**: This is true because diffraction is a phenomenon that occurs when light waves encounter an obstacle or a slit that is comparable in size to their wavelength. This behavior shows that light does not always travel in straight lines, which is a fundamental assumption of the ray model of light. Thus, diffraction indeed highlights the limitations of treating light purely as rays.

### Step 2: Understanding the Reason (R)
The reason states that "A beam of width α starts to spread out due to diffraction after it has travelled a distance (2α/λ)."

- **Explanation**: This statement is partially true. A beam of light does spread out due to diffraction, but the specific distance given (2α/λ) is not universally correct. The spreading of the beam can be described by the angle of diffraction, which depends on the wavelength (λ) and the width of the beam (α). The distance at which significant spreading occurs is related to these parameters, but the exact relationship can vary based on the context.

### Step 3: Conclusion
- The assertion (A) is true.
- The reason (R) is false because while diffraction causes spreading, the specific distance mentioned is not accurate.

### Final Answer:
- Assertion (A) is true.
- Reason (R) is false.