When a monochromatric light passes from vacuum to a denser medium and vice versa. Which of the following characteristics of the light beam does not change ?

Options

A. Velocity
B. Intensity
C. Wavelength
D. Frequency

Answer

To solve the question about the characteristics of monochromatic light that do not change when it passes from vacuum to a denser medium and vice versa, we can analyze the properties of light during refraction.

### Step-by-Step Solution:

1. **Understanding Refraction**:
- When light travels from one medium to another (e.g., from vacuum to a denser medium), it undergoes refraction. This means that its speed and wavelength will change, but we need to identify which characteristic remains constant.

2. **Analyzing Velocity**:
- The speed of light in a vacuum (denoted as \( c \)) is approximately \( 3 \times 10^8 \) m/s. When light enters a denser medium, its speed decreases. The refractive index \( n \) of a medium is defined as \( n = \frac{c}{v} \), where \( v \) is the speed of light in that medium. Since \( n \) changes when moving to a denser medium, the velocity of light also changes. Thus, velocity does not remain constant.

3. **Analyzing Intensity**:
- The intensity of light is related to the amount of light energy passing through a unit area. When light refracts, some of it is reflected at the boundary, which means not all the incident light is transmitted into the new medium. Therefore, the intensity of the refracted light will not equal the intensity of the incident light, indicating that intensity also changes.

4. **Analyzing Wavelength**:
- The wavelength of light is given by the equation \( \lambda = \frac{v}{f} \), where \( f \) is the frequency. When light enters a denser medium, its speed decreases, which leads to a decrease in wavelength. Thus, the wavelength does not remain constant.

5. **Analyzing Frequency**:
- The frequency of light is determined by the source of the light and does not change when light travels from one medium to another. Since frequency is independent of the medium, it remains constant during the transition from vacuum to a denser medium and vice versa.

### Conclusion:
The characteristic of monochromatic light that does not change when it passes from vacuum to a denser medium and vice versa is **frequency**.

### Final Answer:
**Frequency remains constant.**

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