Which statement about the speed of x-ray photons is correct?

Which statement about the speed of x-ray photons is correct?

• A. They travel at the speed of light
• B. They travel slower than light in vacuum
• C. They travel faster than light
• D. They stop in air

Answer: (A)

X-ray photons are massless quanta of electromagnetic radiation, so in empty space they travel at the universal speed limit: the speed of light. That makes the statement that they travel at the speed of light the best choice. In air, their speed is very close to c, reduced only by the medium’s refractive index, but they do not exceed light speed nor stop just because they’re in air. The other options conflict with relativity or describe an incorrect behavior for photons in a common medium.

X-ray photons are massless quanta of electromagnetic radiation, so in empty space they travel at the universal speed limit: the speed of light. That makes the statement that they travel at the speed of light the best choice. In air, their speed is very close to c, reduced only by the medium’s refractive index, but they do not exceed light speed nor stop just because they’re in air. The other options conflict with relativity or describe an incorrect behavior for photons in a common medium.