Which statement about light speed in different media is accurate?

• 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’s speed in a medium is set by the medium’s optical density, described by the refractive index n, with v = c/n. Air has a refractive index very close to 1, while water’s is about 1.33, so light moves about 0.9997c in air versus ~0.75c in water. That means light travels faster in air than in water. Saying this is because air is less dense gives a rough intuition, but the precise reason is the refractive index—the optical density that slows light. The other statements would require speeds that don’t match v = c/n, so they’re not correct.