When a ray of light enters a glass slab from air.

Options

A. its wavelength decreases
B. its wavelength increases
C. its frequency increases
D. neither its wavelength nor its frequency changes

Explanation

To solve the question "When a ray of light enters a glass slab from air, what happens?", we can break it down into several steps:

### Step 1: Understanding the Mediums
- Light travels through different mediums, such as air and glass. Air is less dense compared to glass, which is optically denser.

**Hint:** Remember that the density of a medium affects how light behaves when it passes through it.

### Step 2: Refraction of Light
- When light travels from a less dense medium (air) to a denser medium (glass), it bends towards the normal. This bending of light is known as refraction.

**Hint:** Visualize the normal line (a perpendicular line to the surface at the point of incidence) to understand how the light ray bends.

### Step 3: Speed of Light in Different Mediums
- The speed of light decreases when it enters a denser medium. This is because the molecules in the denser medium interact more with the light waves, causing a reduction in speed.

**Hint:** Consider how the interaction between light and the medium affects the speed of light.

### Step 4: Frequency and Wavelength Relationship
- The frequency of light remains constant when it transitions between mediums. This is because frequency is determined by the source of the light and does not change when entering a different medium.

**Hint:** Think about how frequency is related to the source of the light and is not influenced by the medium.

### Step 5: Change in Wavelength
- Since the speed of light decreases and the frequency remains constant, the wavelength must also decrease. This is because the relationship between speed (v), frequency (f), and wavelength (λ) is given by the equation:
\[ v = f \cdot \lambda \]
If the speed (v) decreases and frequency (f) remains constant, then the wavelength (λ) must decrease.

**Hint:** Use the formula \( v = f \cdot \lambda \) to understand how changes in speed and frequency affect wavelength.

### Conclusion
- Therefore, when a ray of light enters a glass slab from air, its wavelength decreases.

**Final Answer:** The correct option is that the wavelength decreases.