Radiation

Electromagnetic energy travels in the form of waves, moving in straight lines at a speed of 3.00 × 10⁸ meters per second, or 186,400 mi (299,918 km) per second. That speed is usually referred to as the speed of light in a vacuum, because light is the most familiar kind of electromagnetic radiation and because light slows down a little bit when it enters a transparent substance such as glass, water, or air. The speed of light in a vacuum, the velocity of electromagnetic waves, is a fundamental constant of nature.

Electromagnetic radiation can have a variety of energies. Because it travels in the form of waves, the energies are often expressed in terms of wavelengths. The higher the energy of a wave, the shorter its wavelength. The wavelengths of known electromagnetic radiation range from less than 10^-10 centimeter for the highest energies up to millions of centimeters (tens of miles) for the lowest energies.

The energy of a wave can also be expressed by stating its frequency: the number of vibrations or cycles per second. Scientists call one cycle per second a hertz, abbreviated as Hz. Known electromagnetic radiations range in frequency from a few Hz for the lowest energies up to more than 10²⁰ Hz for the highest.