The word photography comes from two ancient Greek words: photo, for “light,” and graph, for “drawing.” “Drawing with light” is a way of describing photography. When a photograph is made, light or some other form of radiant energy, such as X rays, is used to record a picture of an object or scene on a light-sensitive surface. Early photographs were called sun pictures, because sunlight itself was used to create the image. Mankind has been a maker of images at least since the cave paintings of some 20,000 years ago. With the invention of photography, a realistic image that would have taken a skilled artist hours or even days to draw could be recorded in exact detail within a fraction of a second.