The Sun emits ultraviolet radiation at all wavelengths, including the extreme ultraviolet where it crosses into X-rays at 10 nm.
Extremely hot stars emit proportionally more UV radiation than the Sun.
Extreme UV (EUV or sometimes XUV) is characterized by a transition in the physics of interaction with matter.
Wavelengths longer than about 30 nm interact mainly with the outer valence electrons of atoms, while wavelengths shorter than that interact mainly with inner-shell electrons and nuclei.
While optics can be used to remove unwanted visible light that contaminates the VUV, in general, detectors can be limited by their response to non-VUV radiation, and the development of "solar-blind" devices has been an important area of research.
EUV is strongly absorbed by most known materials, but it is possible to synthesize multilayer optics that reflect up to about 50 percent of EUV radiation at normal incidence.
Consequently, the chemical and biological effects of UV are greater than simple heating effects, and many practical applications of UV radiation derive from its interactions with organic molecules.
Suntan and sunburn are familiar effects of over-exposure of the skin to UV, along with higher risk of skin cancer.
It is also produced by electric arcs and specialized lights, such as mercury-vapor lamps, tanning lamps, and black lights.
Although long-wavelength ultraviolet is not considered an ionizing radiation because its photons lack the energy to ionize atoms, it can cause chemical reactions and causes many substances to glow or fluoresce.