Most of the ultraviolet rays that hit the Earth come from the sun. However, there are man-made items that give off ultraviolet rays, such as tanning beds, mercury vapor lamps and black lights.
The wavelengths of ultraviolet rays are between 100 and 400 nanometers. This gives the rays a high amount of energy, which is why they are dangerous to living organisms that are exposed to them for prolonged periods of time. The ultraviolet rays are so strong that they are capable of knocking electrons off of atoms and can even cause them to split. This results in a change to the atom's chemical structure, which can cause cell damage and deformities in the organism.