To solve the problem, we need to analyze how the parameters of a ray of light change when it travels from air into water. The parameters we are considering are wavelength (λ), frequency (n), velocity (v), and intensity (I).
### Step-by-Step Solution:
1. **Understanding the Parameters**:
- In air, the ray of light has a wavelength λ, frequency n, velocity v, and intensity I.
- When the ray enters water, these parameters change to λ', n', v', and I'.
2. **Frequency of Light**:
- The frequency of light (n) is determined by the source and does not change when light passes from one medium to another.
- Therefore, we can write:
\[
n' = n
\]
3. **Velocity of Light**:
- The velocity of light changes when it moves from one medium to another. The speed of light in a medium is given by:
\[
v' = \frac{c}{n_{medium}}
\]
- In this case, the speed of light in water (v') will be less than in air (v).
4. **Wavelength of Light**:
- The relationship between the speed of light, frequency, and wavelength is given by:
\[
v = n \cdot \lambda
\]
- When light enters water, we have:
\[
v' = n' \cdot \lambda'
\]
- Since we established that \( n' = n \), we can relate the wavelengths:
\[
v' = n \cdot \lambda' \quad \text{and} \quad v = n \cdot \lambda
\]
- Thus, we can write:
\[
\frac{v'}{v} = \frac{\lambda'}{\lambda}
\]
5. **Intensity of Light**:
- The intensity of light can change depending on the medium and the reflection at the boundary. However, the total energy must be conserved. This means that some intensity will be reflected and some will be transmitted.
- The relationship between the intensities can be complex and depends on the reflection and transmission coefficients, but generally, we can say that:
\[
I' \leq I
\]
### Conclusion:
From the analysis, the correct relation that we can derive is:
\[
n' = n
\]
This means that the frequency of light remains constant when it passes from air to water.
