Light continuously reaches the earth from the sun. You might also say, "The sun sends electromagnetic energy as light." The electromagnetic waves emitted by the sun are of a broad spectrum ranging from X-rays with a wavelength of 2 nanometers to radio waves with a wavelength of 10 meters. The most intense of these to reach the earth's surface is visible light, with a wavelength around 500 nanometers.

The energy the earth receives from the sun is called the "solar constant," which is defined as 2 calories per square centimeter per minute. Based on meters squared, the solar constant is equivalent to 1.4 kilowatts, or one electric heater. The energy reaching the earth's surface is less than this value, owing to such factors as atmospheric absorption, but the sun is nevertheless a major source of energy for the earth.