When we call LIGHT in usual sense, it means visual light (optical light), which can be seen by naked eyes. In the wide sense, however, the word LIGHT includes unseen light, such as ultraviolet or infrared. Visual light is a kind of electromagnetic waves, whose wavelengths are roughly from 380nm to 770nm. Most of the electromagnetic wave emitted from the sun is the visual light. Moreover, the Earth's atmosphere is transparent to the visual light. As a result, the life form on the earth may evolve so as to be sensitive to the visual light.

Atmospheric Window

Electromagnetic waves are roughly classified as radio, infrared, visual, ultraviolet, X-ray, and gamma-ray in the order of wavelength. Most of electromagnetic waves from celestial objects are absorbed in the atmosphere surrounding the Earth, and only small fraction can reach the ground. The wavelength regions of electromagnetic wave, where the electromagnetic wave can pass through the atmosphere and we can observe the universe, are called atmospheric window. The radio waves with long wavelengths are reflected by the ionosphere surrounding the Earth. The radio waves with wavelength of millimeter to meter can pass throught the atmosphere without absorption. Near infrared and millimeter waves are absorbed by water vapor and carbon dioxide in the upper atmosphere. The atmosphere is transparent to the visual light with wavelength of 300nm to 1000nm. Ultraviolet light is absorbed by ozone and other molecules in the atmosphere, X-rays by atoms, and gamma-rays by various nuclei. As an atmospheric window, ultimately, there are radio window with wavelength of 1 millimeter to 30 meter and optical window with wavelength of 300 nanometer to 1000 nanometer.

Wavelength and Frequency Electromagnetic Wave and Photon Unit