Which statement about the speed of light is true?

Which statement about the speed of light is true?

• A. It travels at the speed of light in all media
• B. It travels faster in water than in vacuum
• C. It travels at the speed of light in vacuum
• D. It slows to zero in glass

Answer: (C)

Light in vacuum moves at a universal speed, c, about 3.00 x 10^8 m/s. This speed is a constant of nature and sets the fastest possible rate for information transfer. In any material, light slows down because it interacts with the atoms in the medium, causing delays as photons are absorbed and re-emitted or otherwise scattered. The speed in a medium is given by v = c/n, where n is the refractive index; since most materials have n > 1, light travels more slowly than c there. It does not travel faster in water than in vacuum, nor does it stop in glass under normal conditions. So the true statement is that light travels at the speed of light in vacuum.

Light in vacuum moves at a universal speed, c, about 3.00 x 10^8 m/s. This speed is a constant of nature and sets the fastest possible rate for information transfer. In any material, light slows down because it interacts with the atoms in the medium, causing delays as photons are absorbed and re-emitted or otherwise scattered. The speed in a medium is given by v = c/n, where n is the refractive index; since most materials have n > 1, light travels more slowly than c there. It does not travel faster in water than in vacuum, nor does it stop in glass under normal conditions. So the true statement is that light travels at the speed of light in vacuum.