Considering different media, in which of the following does light travel slowest?

Introduction / Context:
The speed of light depends on the medium through which it travels. In a vacuum, light travels at its maximum speed, while in materials like air, water and glass it slows down to different degrees. This question asks you to identify the medium among the options in which light travels the slowest, which is closely related to the refractive index of each medium.

Given Data / Assumptions:
- The media considered are glass, water, air and vacuum.
- We assume common types of glass and pure water at standard conditions.
- Refractive index n is used to relate the speed of light in a medium to its speed in vacuum.

Concept / Approach:
The speed of light in a medium is given by v = c / n, where c is the speed of light in vacuum and n is the refractive index of the medium. A higher refractive index means light travels more slowly. Vacuum has n = 1 and provides the maximum possible speed, c. Air has a refractive index very close to 1, so light travels almost as fast as in vacuum. Water has a refractive index around 1.33, so light is slower there. Common glass has a refractive index typically around 1.5 or higher, meaning light slows down even more. Therefore, among the given options, light is slowest in glass.

Step-by-Step Solution:
Step 1: Recall the relationship v = c / n between speed v in a medium and refractive index n. Step 2: Note that n for vacuum is exactly 1, so v = c in vacuum, the highest possible speed. Step 3: Air has n slightly greater than 1 (about 1.0003), so light speed in air is only slightly less than c. Step 4: Water has n about 1.33, so light is slower in water than in air. Step 5: Typical glass has n about 1.5 or more, so v = c / 1.5, which is less than the speed in water. Step 6: Therefore, among glass, water, air and vacuum, light travels slowest in glass.

Verification / Alternative check:
You can also reason qualitatively: the denser the optical medium (in terms of refractive index, not just mass density), the more light interacts with the material and the more it slows down. Data tables of refractive indices confirm that glass usually has a higher refractive index than water, while air is close to vacuum. Lenses made of glass bend light more strongly precisely because light is slower in glass, supporting the idea that its refractive index is higher than that of water or air.

Why Other Options Are Wrong:
Water: Light is slower in water than in air or vacuum but faster than in most types of glass, which have higher refractive indices.
Air: Light travels very close to its vacuum speed in air, so it is not the slowest medium listed.
Vacuum: This is the medium where light is fastest, with speed c; it is the opposite of the correct choice.

Common Pitfalls:
Students sometimes confuse mass density with optical density. While they are often correlated, the key quantity for light speed is refractive index. Another mistake is to think that water must slow light more than glass because it is a liquid. In reality, glass is optically denser than water in most common cases. Remember that the larger the refractive index n, the smaller the speed v = c / n, and that typical values place glass above water in refractive index.

Final Answer:
Light travels slowest in glass among the media listed.