The visible surface of the Sun is at a temperature of no more than 6000 K, but parts of its atmosphere have a temperature of over 1 000 000 K. It emits radiation in all regions of the electromagnetic spectrum but overall its spectrum resembles that of a black body at 5800 K. We can divide the spectrum into visible light with a wavelength between 400 and 760 nm (which we can see), infra-red (which has a longer wavelength than visible light, for example the heat from an electric fire) and ultra-violet with a wavelength between 240 and 400 nm. Approximately 7% of the energy output is in the ultra-violet, 41% in the visible and 52% in the infra-red. The ultra-violet part of the spectrum can be further divided into UV-A, UV-B and UV-C. UV-A lies between 400 and 320 nm and gives rise to a sun-tan and ageing of the skin. UV-B lies between 320 and 280 nm and is the damaging part of the spectrum. UV-C lies between 280 and 240 nm and is totally absorbed in the atmosphere before it can reach the ground. The Dobson spectrophotometer uses wavelengths between 305 and 340 nm to measure the amount of ozone in the atmosphere.