hen a ray of light enters a glass slab from air:

The correct option is: its wavelength decreases

Step-by-step Explanation:

1. Understanding the Medium Transition:
When a ray of light travels from air to a glass slab, it transitions from an optically rarer medium (air, with a refractive index of approximately 1.0) to an optically denser medium (glass, with a refractive index of approximately 1.5).

2. Behavior of Frequency:
The frequency of light depends only on its source of generation and does not depend on the medium through which the light travels. Therefore, when light enters glass from air, its frequency remains unchanged.

3. Behavior of Speed:
Since glass is optically denser than air, the speed of light decreases as it enters the glass slab. The speed of light in a medium is given by the formula:

$v=\frac{c}{n}$

where $v$ is the speed of light in the medium, $c$ is the speed of light in vacuum/air, and $n$ is the refractive index of the medium. Since the refractive index of glass is greater than 1, the speed of light decreases inside the glass.

4. Relationship between Speed, Frequency, and Wavelength:
The speed of light, its wavelength, and its frequency are related by the wave equation:

$v=f\cdot \lambda$

Rearranging this equation to express wavelength, we get:

$\lambda =\frac{v}{f}$

Since the frequency ($f$) remains constant and the speed ($v$) decreases, the wavelength ($\lambda$) must directly decrease to balance the equation. Therefore, as light enters the glass slab, its wavelength decreases.