When light passes from one medium into another medium, then the physical property which does not change is:

Options

A. Velocity
B. Wavelength
C. Frequency
D. Refractive index

Answer

To solve the question, "When light passes from one medium into another medium, then the physical property which does not change is:", we will analyze the behavior of light as it transitions between different media.

### Step-by-Step Solution:

1. **Understanding Refraction**:
- When light travels from one medium to another (e.g., from air to water), it undergoes a phenomenon called refraction. This bending of light occurs due to a change in its speed as it enters a different medium.

2. **Identifying Properties of Light**:
- The key properties of light to consider during refraction are:
- Speed of light (v)
- Wavelength (λ)
- Frequency (ν)
- Refractive index (n)

3. **Effect of Medium Change on Speed**:
- The speed of light changes when it moves from one medium to another. For example, light travels faster in air than in water. Therefore, the speed of light does change.

4. **Effect on Wavelength**:
- The relationship between speed, frequency, and wavelength is given by the equation:
\[ v = ν \cdot λ \]
- Since the speed of light changes when it enters a different medium, the wavelength also changes accordingly. Thus, wavelength does change.

5. **Effect on Frequency**:
- Frequency is defined as the number of waves that pass a point in one second. Importantly, frequency remains constant when light passes from one medium to another. This is because frequency is determined by the source of the light and does not depend on the medium.

6. **Effect on Refractive Index**:
- Each medium has its own refractive index, which is a measure of how much light slows down in that medium. When light transitions from one medium to another, the refractive index changes. Therefore, the refractive index does change.

7. **Conclusion**:
- Among the properties discussed, the only property that does not change when light passes from one medium to another is frequency.

### Final Answer:
The physical property that does not change when light passes from one medium into another is **frequency**.