Figure 1 The spectrum of solar radiation that is incident on the top of the Earth's atmosphere, showing the bands of the ultraviolet that are referred to in the text. The human eye can see only the region labeled "visible light," which is composed of the familiar rainbow of colors from violet to red. The ultraviolet begins where the violet response of our eye ends. In units of wavelength, the UVC extends from 100 to 280 nanometers (1 nm= 1 billionth of a meter); the UVB, which is less energetic, from 280 to 315 nm; and the least energetic, UVA, from 315 nm to the beginning of visible light, about 400 nm. UV radiation that forms ozone in the upper atmosphere constitutes only a portion of the UVC, from about 200 to 250 nm.