Ultraviolet light

Section 1 of 4

Sunlight holds more than the colours you can see. Just beyond violet lies ultraviolet light, the part of sunlight that burns skin.

Ultra means beyond. Ultraviolet light, or UV, has waves shorter than violet's 400 nm, too short for your eyes to see. Light arrives in tiny packets of energy called photons, and the shorter the wave, the more energy each photon carries. A photon of UV at 300 nm carries more than twice the energy of a photon of red light at 700 nm: enough to break some bonds and start reactions.

Scientists split ultraviolet light into three bands. UVC has the shortest waves and the most energy, then comes UVB, then UVA, next to violet. The ozone layer high in the air absorbs all the UVC and most of the UVB, so about 95% of the UV that reaches the ground is UVA.

How much of each kind of ultraviolet light reaches the ground Three rows, one for each kind of ultraviolet light, each drawn as a wavy arrow from the left. UVC, 100 to 280 nanometres, has the shortest waves, and its arrow stops at the ozone layer. UVB, 280 to 315 nanometres, has longer waves: its arrow reaches the ozone layer and only a thin wave carries on to the ground, marked a little. UVA, 315 to 400 nanometres, has the longest waves, and its arrow passes the ozone layer at full strength and reaches the ground, marked most. The UVC row is marked none. ozone layer ground UVC 100 to 280 nm none UVB 280 to 315 nm a little UVA 315 to 400 nm most
The three bands of ultraviolet light. Shorter waves carry more energy in each photon. The ozone layer absorbs all of the UVC and most of the UVB, and most of the UVA passes through.

What UV does to skin

UVB is the main cause of sunburn. The bases of DNA, the flat rings that pair up across the double helix, absorb UV light, including UVB. When two thymine bases sit side by side on one strand, a photon can open the C=C double bond in each, and the two rings join through a new ring of four carbon atoms. The joined pair puts a kink in the DNA.

Two neighbouring thymines join when they absorb UVB Two panels, each showing two thymine bases side by side on one strand of DNA, drawn as skeletal formulas and joined at the bottom to the backbone. Each thymine is a six-membered ring with two nitrogen atoms, two C=O groups and a methyl group. In the top panel, the facing edge of each ring is a highlighted C=C double bond. An arrow labelled UVB leads to the bottom panel, where both of those double bonds have become single bonds and two new highlighted bonds join the rings, making a square ring of four carbon atoms between them. OONNHOONNH two thymines side by side backbone OONNHOONNH after UVB: joined by a ring backbone UVB
Two thymine bases side by side on one strand of DNA. UVB opens the highlighted C=C double bond in each, and the rings join through a square of four carbon atoms. The short line going straight up from each ring is a methyl group, -⁠CH₃.

Your cells have repair enzymes that cut out damaged stretches of DNA and rebuild them, and they fix most of this damage. Sunburn, the red, sore skin that shows up a few hours after too much sun, is the skin's response to it. A joined pair that is copied before it is repaired can change the DNA's instructions, and over many years changes like these can lead to skin cancer.

UVA carries less energy in each photon, and it reaches deeper into the skin. It harms DNA too, both by joining thymines as UVB does and by making reactive forms of oxygen that attack it, and it is the main cause of the wrinkles and dark spots of skin aged by the sun.

Quick check

Which of these carries the most energy in each photon?