A : If a wave moving in a rarer medium , gets reflected at the boundary of a denser medium , then it encounter a sudden change in phaso or `pi` .
R : If a wave propagating in a denser medium , gets reflected from rarer medium , then there will be no abrupt phase change .

Answer

To analyze the given statements about wave reflection at the boundary of different media, we will break down the assertions and reasons step by step.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that if a wave moving in a rarer medium gets reflected at the boundary of a denser medium, it encounters a sudden change in phase of π (180 degrees).
- In wave mechanics, when a wave travels from a rarer medium (lower density) to a denser medium (higher density), it reflects with a phase change of π.

2. **Explaining the Reflection Process**:
- Consider a wave traveling in a rarer medium (Medium 1) and reaching the boundary of a denser medium (Medium 2).
- When the wave hits the boundary, it reflects back into the rarer medium.
- The reflected wave can be represented mathematically as:
\[ y_r = -A \sin(kx - \omega t) \]
- This indicates that the amplitude is reversed (indicated by the negative sign), which corresponds to a phase shift of π.

3. **Conclusion on Assertion (A)**:
- Since the reflected wave undergoes a phase change of π, the assertion is **correct**.

4. **Understanding the Reason (R)**:
- The reason states that if a wave propagating in a denser medium gets reflected from a rarer medium, there will be no abrupt phase change.
- When a wave travels from a denser medium to a rarer medium, it reflects without a phase change.

5. **Explaining the Reflection Process for Reason (R)**:
- When the wave in the denser medium (Medium 2) reaches the boundary of the rarer medium (Medium 1), it reflects back into the denser medium without any phase shift.
- The reflected wave can be represented as:
\[ y_r = A \sin(kx - \omega t) \]
- Here, the amplitude remains the same, and there is no negative sign, indicating no phase change.

6. **Conclusion on Reason (R)**:
- Since there is no phase change when reflecting from a denser to a rarer medium, the reason is also **correct**.

### Final Conclusion:
- Both the assertion (A) and the reason (R) are correct. The assertion correctly describes the behavior of waves reflecting from a denser medium, and the reason accurately explains the behavior of waves reflecting from a rarer medium.