Ultraviolet (UV) light is electromagnetic radiation with wavelengths from 10 to 400 nm. This light is invisible to human eyes. It has shorter wavelengths than visible light, but longer than X-rays. It is energetic enough to damage human skin, sterilize surfaces from bacteria, and break molecular bonds. About 10% of the light given off by the sun is in the Ultraviolet frequencies.
Subtypes (Colors) of Ultraviolet (UV)
Ultraviolet A: is from 315 to 400 nm. Most of this gets thru the Ozone layer. "Soft UV".
Ultraviolet B: is from 280 to 315 nm. Most of this IS absorbed by the Ozone layer. "Medium UV".
Ultraviolet C: is from 100 to 280 nm. Completely absorbed by the Ozone layer. "Hard UV".
Extreme UV shades into the lower energy X-rays. This ranges from 10 to 120 nm. This light can break molecular bonds.
UV Light on Mars
Mars gets about 50% of the light from the Sun, so while the UV light is just as powerful per photon, there is half as much light. This powerful light will sterilize bacteria on the surface of the planet. If humans could somehow give Mars an oxygen atmosphere, then an Ozone Layer would build up on Mars which would protect the surface from this radiation.
A concern is that UV light will break up Super Greenhouse Gases which people have suggested could warm the planet.
UV Light Effects On Life
Most humans can not see wavelengths shorter than 400 nm. Some insects, birds, and fish can see UV light down to ~300 nm.
UV light will damage skin. Life has either created barriers (such as fur) to protect against it, or evolved to have melanin in their shells or skin to prevent it from penetrating deeply into the body.
Long exposure to UV light can harm the eyes.
UV-B light helps generate vitamin D in human skin, so people living inside for long period of times may wish to have UV lights in the bathroom to help them get enough of these frequencies.
Bright lights and UV light promotes the creation of Serotonin (a brain chemical), which provides sensations of well-being, serenity, and happiness.
Material Science / Industrial Uses
UV light can check for fires.
UV light can be used in photolithography where a mask is used to shadow some areas from the light, while others are exposed. Chemical reactions happen to the exposed areas
UV light can cure certain polymers. Some 'glues' can harden under UV light.