A : Longitudinal waves do not exhibit the phenomenon of polarisation .
R : In longitudinal waves medium particle vibrate in direction normal to the wave propagation .

Answer

To solve the question, we need to analyze both the assertion (A) and the reason (R) provided.

### Step 1: Analyze the Assertion (A)
The assertion states that "Longitudinal waves do not exhibit the phenomenon of polarization."

- **Explanation**: This statement is correct. Polarization is a phenomenon that occurs in transverse waves, where the vibrations of the medium are perpendicular to the direction of wave propagation. Longitudinal waves, on the other hand, involve particle motion in the same direction as the wave propagation, which means they cannot be polarized.

### Step 2: Analyze the Reason (R)
The reason states that "In longitudinal waves, medium particles vibrate in direction normal to the wave propagation."

- **Explanation**: This statement is incorrect. In longitudinal waves, the medium particles vibrate in the same direction as the wave travels, not in a direction normal (perpendicular) to it. For example, in a sound wave, air molecules compress and rarefy in the direction of the wave's travel.

### Step 3: Conclusion
- Since the assertion is correct and the reason is incorrect, we conclude that the overall statement is true. The assertion is true, but the reason given for it is false.

### Final Answer
- **Assertion (A)**: True
- **Reason (R)**: False