Ultraviolet light is a form of radiation which is not visible to the human eye. The type of radiation being emitted depends on the temperature of the object. A coal glowing red in a barbecue is cooler than our Sun, which appears yellow, which is cooler still than some stars which appear bright white. Visible light runs from the familiar blue to green to yellow to orange to red.
Red light is the least energetic of the colors of visible light, and blue is the most energetic.
Beyond the red end of the visible part of the spectrum lies the infrared and radio radiation. Many scientists are interested in studying the invisible universe of ultraviolet light, since the hottest and the most active objects in the cosmos give off large amounts of ultraviolet energy. The near ultraviolet , abbreviated NUV, is the light closest to optical or visible light. It has wavelengths from about 1,800 to 3,000 angstroms . The extreme ultraviolet , abbreviated EUV, is the ultraviolet light closest to X-rays, and is the most energetic of the three types.