In basic optics, the speed of light is minimum when it passes through which one of the following media?

Introduction / Context:
The speed of light is a fundamental concept in physics and basic science examinations. In a vacuum, light travels at its maximum speed, but in different media such as air, water, or glass, its speed changes because of the optical density of the medium. This question tests your understanding of how the speed of light varies with different transparent materials and the idea of refractive index and optical density.

Given Data / Assumptions:

• We are comparing the speed of light in four media: vacuum, air, water, and glass.
• The refractive index of each medium is different.
• There are no special experimental conditions; standard conditions are assumed.
• Light is passing normally through each medium.

Concept / Approach:
The key concept is the relation between speed of light and refractive index. The refractive index n of a medium is defined as n = c / v, where c is the speed of light in vacuum and v is the speed of light in the medium. A higher refractive index means light travels more slowly in that medium. Vacuum has refractive index 1, air has a value slightly greater than 1, water has a larger refractive index than air, and glass usually has a refractive index higher than that of water. Therefore, the speed of light is slowest in the medium with the highest refractive index.

Step-by-Step Solution:
Step 1: Recall that in vacuum, light has its maximum speed and refractive index is minimum (approximately 1). Step 2: Air is only slightly denser optically than vacuum, so the speed of light in air is almost the same as in vacuum but still slightly lower. Step 3: Water is denser than air and has a higher refractive index, so light travels more slowly in water than in air. Step 4: Glass is typically more optically dense than water and has a higher refractive index than water. Step 5: Since speed v = c / n, the highest value of n gives the smallest value of v. Among the given options, glass has the highest refractive index. Step 6: Therefore, the speed of light is minimum while passing through glass.

Verification / Alternative check:
Another way to remember this is the common ordering used in school physics: speed of light is greatest in vacuum, slightly less in air, less again in water, and least in denser transparent solids like glass and some crystals. Textbooks and exam tables usually show refractive index values around n(air) ≈ 1.0003, n(water) ≈ 1.33 and n(glass) ≈ 1.5 (values vary slightly with type of glass). This confirms that glass has the highest refractive index among the listed media and hence the minimum speed of light.

Why Other Options Are Wrong:
Vacuum: Light travels fastest in vacuum, not slowest, so vacuum cannot give minimum speed.
Air: Air is only slightly denser than vacuum, so the speed of light is still high in air compared to liquids and solids.
Water: Although light travels slower in water than in air, water is still usually less optically dense than common glass, so the speed is not minimum in water among these options.

Common Pitfalls:
Many learners incorrectly think that water provides the minimum speed because it is a liquid and seems denser than glass in everyday experience. Others confuse the idea of mechanical waves, which may behave differently, with electromagnetic waves such as light. Another common mistake is to assume that vacuum slows light, when in fact vacuum provides the maximum speed. It is essential to connect this topic with refractive index and remember that higher refractive index implies lower speed of light in that medium.

Final Answer:
The speed of light is minimum when it passes through Glass among the given options.