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 solve the problem, we will analyze both the assertion (A) and the reason (R) provided in the question.

### Step 1: Understand the Assertion (A)
The assertion states that if a wave traveling in a rarer medium is reflected at the boundary of a denser medium, it encounters a sudden change in phase of π (180 degrees).

- **Explanation**: When a wave travels from a rarer medium (like air) to a denser medium (like water), upon reflection at the boundary, there is a phase change of π. This is because the wave reflects off a medium that is denser, leading to an inversion of the wave.

### Step 2: Analyze the Reason (R)
The reason states that if a wave propagating in a denser medium is reflected from a rarer medium, there will be no abrupt phase change.

- **Explanation**: When a wave travels from a denser medium to a rarer medium, it reflects without any phase change. This means that the wave retains its original phase upon reflection, as it is moving from a medium of higher density to one of lower density.

### Step 3: Determine the Truth of Both Statements
- **Assertion (A)**: True. The wave does experience a phase change of π when reflecting from a denser medium.
- **Reason (R)**: True. The wave does not experience a phase change when reflecting from a rarer medium.

### Step 4: Evaluate the Relationship Between A and R
While both the assertion and the reason are true, the reason does not explain the assertion. The assertion discusses the behavior of waves reflecting from a denser medium, while the reason discusses the opposite scenario.

### Conclusion
Both statements are true, but the reason does not correctly explain the assertion. Therefore, the correct answer is that both A and R are true, but R is not the correct explanation of A.

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