Light-year: A unit of astronomical distance equivalent to the distance that light travels in one year, which is 9.461 trillion kilometers or about 5.879 trillion miles.
Electromagnetic spectrum: The range of all types of electromagnetic radiation, including visible light, radio waves, and X-rays. The speed of light applies to all these forms of radiation.
Photon:This is a particle representing a quantum of light or other electromagnetic radiation. It carries energy proportional to the radiation frequency but has zero rest mass.