The speed of light is maximum when it travels in which of the following media?

Introduction / Context:
Light and other electromagnetic waves can travel through different media, such as vacuum, air, water, and glass. The speed of light is not the same in all media; it depends on the optical properties of the material. This question asks you to identify the medium in which the speed of light attains its maximum value.

Given Data / Assumptions:

• We are comparing the speed of light in vacuum, solids, liquids, and gases.
• The fundamental constant c denotes the speed of light in vacuum.
• When light enters a material medium, its speed decreases compared with c.
• We are not considering exotic media with special properties, only typical physical states.

Concept / Approach:
The speed of light in vacuum is a fundamental constant of nature, usually denoted by c and approximately equal to 3 * 10^8 m/s. When light passes through a material medium, its speed is reduced. The ratio of c to the speed of light in the medium is called the refractive index. For any transparent material, the refractive index is greater than or equal to 1, so the speed of light in that material is less than or equal to c. Among the listed options, only vacuum has refractive index exactly 1 and allows light to travel at the maximum speed c.

Step-by-Step Solution:
Step 1: Recall that in vacuum, light travels at speed c, which is approximately 3 * 10^8 m/s. Step 2: In air, which is a gas, the speed of light is slightly less than c because the refractive index of air is just above 1, though very close to 1. Step 3: In liquids such as water and solids such as glass, the refractive index is greater than that of air, so the speed of light is further reduced. Step 4: Since speed decreases as refractive index increases, the maximum speed is achieved in the medium with the lowest refractive index, which is vacuum. Step 5: Therefore, the speed of light is maximum in vacuum, compared with solids, liquids, or gases.

Verification / Alternative check:
Typical refractive index values are about 1 for vacuum, approximately 1.0003 for air, about 1.33 for water, and around 1.5 for common glass. The speed of light in any medium v is given by v = c / n, where n is the refractive index. Substituting these values shows that v is very slightly less than c in air and significantly less in water and glass. This confirms that light travels fastest in vacuum and progressively slower in gases, liquids, and solids.

Why Other Options Are Wrong:
Solids: Have refractive indices greater than 1, so light travels slower than in vacuum. Liquids: Also have refractive indices greater than 1 and slow light more than air does. Gases: Most gases have refractive indices slightly greater than 1; light speed is close to but still less than c.

Common Pitfalls:
Some learners assume that light travels fastest in air because it seems to move very freely there, but they forget that air is still matter, not vacuum. Others confuse sound and light, thinking that because sound travels fastest in solids, light might also. In reality, sound is a mechanical wave requiring a medium, while light is an electromagnetic wave that needs no medium and achieves its maximum speed in vacuum.

Final Answer:
The speed of light is maximum when it travels in a vacuum.