Ultraviolet

Ultraviolet (also called UV) is between visible light and X-rays in the electromagnetic spectrum:

The wavelength range is about 380 nanometers (380 billionths of a meter) to 10 nanometers.

sun dog

Ultraviolet is invisible to the human eye.

The Sun sends out UV in all wavelengths, but only some get through the atmosphere.

The ranges from lowest to highest energy are:

UVA

UVB

sun feet

UVC

sunscreen

Sun Safe

So UV is mostly bad for us, except for the benefit of Vitamin D.

On balance we should be careful how much UV exposure we get, using sunscreen (one that blocks both UVA and UVB), sunglasses, wide hats and protective clothing.

Extreme Ultraviolet

Beyond UVC is "extreme ultraviolet" (10 to 121 nanometers). It is ionizing, meaning it has such high energy it can knock electrons out of atoms, which is very harmful to our cells, killing them or changing them so they grow out of control and form cancer.

Thankfully, extreme UV is also totally absorbed by the atmosphere.

Uses

UV has many uses in science, medicine and industry including:

uv irradiation
Killing germs with ultraviolet in a Lab