Which statement about light speed in different media is accurate? (Second version)

• A. Light travels faster in water than in air.
• B. Light travels at the same speed in air and water.
• C. Light travels slower in air than in water because air is thin.
• D. Light travels faster in air than in water because air is less dense.

Answer: (D)

Light travels at different speeds in different media because optical density (refractive index) of the material slows the light. The speed in a medium is roughly c divided by its refractive index. Air has a refractive index very close to 1 (about 1.0003), while water is around 1.33, so light moves much faster in air than in water. That’s why the statement is correct: light travels faster in air than in water, largely because air is less optically dense. The other ideas don’t fit because light isn’t slowed by “thinness,” and water is not faster than air—it's slower due to its higher refractive index. In numbers, light is about 299,800 km/s in air and about 225,000 km/s in water.