A: When monochromatic light is incident on a surface separating two media, the reflected and refracted light both have the same frequency as the incident frequency.
R: At any interface between the two media , the electric (and magnetic) fields must satisfy certain boundary conditions for all times and frequency determines the time dependence of fields.

Options

A. IF both Assertion & Reason are true and the reason is the correct explanation of the assertion, then mark (1).
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion, then mark(2)
C. IF Assertion is true statement but Reason is false then mark(3)
D. If both Assertion and Reason are false statements, then mark (4)

Answer

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

### Step 1: Understanding the Assertion (A)
The assertion states that when monochromatic light (light of a single frequency) is incident on a surface separating two media, both the reflected and refracted light have the same frequency as the incident light.

- **Explanation**: Monochromatic light means that the light has a single frequency. When this light strikes the boundary between two different media, some of it is reflected back into the first medium, and some is transmitted into the second medium (refracted). The frequency of the light is determined by the source of the light, and it remains constant regardless of the medium through which it travels. Thus, both the reflected and refracted light will have the same frequency as the incident light.

### Step 2: Understanding the Reason (R)
The reason states that at any interface between two media, the electric and magnetic fields must satisfy certain boundary conditions, and frequency determines the time dependence of these fields.

- **Explanation**: When light travels from one medium to another, the electric and magnetic fields associated with the light wave must meet specific boundary conditions at the interface. These conditions ensure that the fields are continuous across the boundary. The frequency of the light wave is related to how these fields oscillate over time. Since the frequency of the incident light does not change when it reflects or refracts, the electric and magnetic fields maintain their frequency as well.

### Step 3: Conclusion
- **Assertion (A)** is correct because the frequency of the light does not change when it reflects or refracts.
- **Reason (R)** is also correct as it explains the behavior of the electric and magnetic fields at the interface.

However, the reason does not fully explain the assertion, as it focuses more on the boundary conditions rather than the constancy of frequency. Therefore, the answer to the question is that both the assertion and reason are correct, but the reason does not adequately justify the assertion.

### Final Answer
- The answer to the question is option 2: Both A and R are correct, but R is not the correct explanation of A.