Light energy is a form of electromagnetic radiation. Light consists of photons, which are produced when an object's atoms heat up. Light travels in waves and is the only form of energy visible to the human eye. Electromagnetic radiation occurs over an extensive range of wavelengths, from gamma rays with wavelengths less than about 1 × 10 - 11 meters to radio waves measured in meters. Within that broad-spectrum, the wavelengths visible to humans occupy a very narrow band, from about 700 nanometers (nm; billionths of a meters) for red light down to about 400 nm for violet light. The speed of light in a vacuum is a fundamental physical constant, the currently accepted value of which is exactly 299,792,458 meters per second, or about 187,282 miles per second.