The velocity of light is the highest in

Concept: The speed of light in a medium is governed by that medium's optical density, expressed through its refractive index n. The relationship is v = c / n, where c is the speed of light in vacuum (about 3 × 10⁸ m/s) and n ≥ 1 for every physical medium. A larger refractive index means a denser medium and a slower light speed.

Apply v = c / n to each medium using standard refractive-index values, then compare:

• Vacuum: n = 1 exactly, so v = c, the maximum possible speed.

• Air: n ≈ 1.0003, so light is only fractionally slower than c.

• Water: n ≈ 1.33, so v ≈ c / 1.33, noticeably slower.

• Glass: n ≈ 1.5, so v ≈ c / 1.5, the slowest of the four.

Cross-check: ordering the media by increasing refractive index gives vacuum (1) < air (1.0003) < water (1.33) < glass (1.5); since speed is inversely proportional to n, the speed order is exactly reversed, confirming vacuum gives the greatest speed and glass the least.