Ultraviolet

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

images/em-spectrum.js

The wavelength range is about 400 nanometers (400 billionths of a meter) to 10 nanometers.

Dog lying outside in the bright sunlight

Ultraviolet is invisible to the human eye.

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

Shorter wavelengths have higher energy. So as the wavelength gets smaller, the UV rays become more energetic and dangerous.

From lowest to highest energy we have:

UVA

Ultraviolet A (315 to 400 nanometers) gets through our atmosphere
Leads to premature aging of skin
Can lead to some types of cancer
Nickname: "UV Aging"

UVB

Ultraviolet B (280 to 315 nanometers) is partly absorbed by the atmosphere
UVB helps our bodies produce Vitamin D, which is good for us
But too much UVB can cause sunburn, damage to our eyes, and is the most common cause of skin cancer
Nickname: "UV Burning"

Sunburned feet showing red skin and pale sandal strap lines

UVC

Ultraviolet C (100 to 280 nanometers) is completely absorbed by the atmosphere
Is very dangerous to us
But since none gets through to us from the Sun, it isn't a problem

Bottle of sunscreen lotion for UV skin protection

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.

Ozone Layer

The Earth's ozone layer (high up in the atmosphere) is what absorbs most of this dangerous high-energy UV light before it reaches the ground.

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:

disinfection
decontamination of water
killing cancer cells (using careful medical procedures!)
imaging of cells
forensic analysis
drug detection
light therapy in medicine
bug zappers (flies are attracted to some UV light)
special glues that harden with UV
and more

Ultraviolet light glowing inside a laboratory sterilizer
Killing germs with ultraviolet in a Lab

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