When a light wave travels from air to glass :

Options

A. its wavelength decreases.
B. its wavelength increases.
C. there is no change in wavelength.
D. its frequency decreases.

Explanation

To solve the question regarding what happens to the wavelength of a light wave when it travels from air to glass, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Medium Change**: When light travels from air (a less dense medium) to glass (a denser medium), it undergoes a change in speed.

2. **Speed of Light in Different Mediums**: The speed of light in air is approximately \( c \) (the speed of light in vacuum), while in glass, the speed of light is less than \( c \). This is due to the refractive index of glass being greater than that of air.

3. **Frequency of Light**: The frequency (\( f \)) of light is determined by the source of the light and does not change when light moves from one medium to another. Therefore, as light enters glass, its frequency remains constant.

4. **Relationship Between Speed, Wavelength, and Frequency**: The relationship between the speed of light (\( c \)), wavelength (\( \lambda \)), and frequency (\( f \)) is given by the equation:
\[ c = \lambda f \]
When light enters a different medium, this relationship still holds, but since the speed of light decreases in glass, we can express it as:
\[ v = \lambda' f \]
where \( v \) is the speed of light in glass and \( \lambda' \) is the new wavelength in glass.

5. **Analyzing the Change in Wavelength**: Since the frequency remains constant and the speed of light decreases, we can rearrange the equation to find the new wavelength:
\[ \lambda' = \frac{v}{f} \]
Since \( v < c \) (the speed of light in glass is less than in air), it follows that \( \lambda' < \lambda \). Thus, the wavelength decreases when light enters glass.

6. **Conclusion**: Therefore, when a light wave travels from air to glass, the wavelength decreases.

### Final Answer:
The wavelength of the light wave decreases when it travels from air to glass.