Charlock in UV light. Coloured photograph of charlock (Sinapis arvensis) flowers showing the bull's-eye pattern of absorbed and reflected ultraviolet (UV) light on the flower heads. These patterns are known as 'nectar guides' and help to attract pollinating insects, which can see UV light. Humans are unable to see these patterns. This image was taken using a digital camera adapted to filter and detect UV light. The resulting monochrome image was coloured on a computer. Areas that absorb UV light are dark in colour. UV light is outside the visible spectrum and does not have a true colour. See image B836/2130 for daylight version.