Which statement best describes how light travels through different phases of matter?

• A. Light slows down in air and speeds up in water because air is less dense.
• B. Light travels at the same speed in all materials.
• C. Light slows down in air and speeds up in water because air is more dense.
• D. Light travels faster through air than through denser media because air is less dense.

Answer: (D)

Light’s speed in a medium is set by the medium’s optical density (refractive index). In vacuum, light is fastest; in any material, it slows down by a factor roughly equal to the inverse of the refractive index. Air has a refractive index very close to 1, so light travels nearly at c, while water and other denser media slow it down more. That’s why light moves faster through air than through denser media—the air’s lower optical density allows the wave to propagate with fewer interactions. The other statements either reverse the density relation or claim no difference, which doesn’t match how light slows in more optically dense substances.