Shadows

Since ancient times people have known that light travels in straight lines. This can be seen by looking at the beam of a light from a film projector. Each ray of light travels in a straight line from the projector to the screen. If someone stands up and blocks part of the light, some of the light will not reach the screen and result in an area without light - a shadow. The smaller the angle between the direction of the light and the surface on which the shadow occurs, the longer the shadow is. If the object is close to the light source, the shadow is large.
Sundials found in ancient Egypt show that the principle of the shadow has been used to tell the time for thousands of years. The sun moves across the sky at a steady rate so if a stick is pushed vertically into the ground, the time of day can be seen by where the shadow lies.
Leonardo da Vinci shows studies of object and shadow in his notebooks and the German mathematician and astronomer Johannes Kepler published a book called Astronomiae pars Optica in 1604 which describes how light cast shadows. In the early 18th Century Etienne de Silhouette, a French government minister used the principle of light rays fanning out from a candle in straight lines to cast a shadow of anything that blocks its path to make shadow portraits that were cheaper than paintings. The name silhouette is now used to describe any black shape seen against light.
The Brockenspeckter is a phenomenon that occurs in high mountains when a climbers shadow falls on the clouds because the sun is in a low position behind the climber. If conditions are right, coloured rings are seen around the shadow. This optic effect gets its name from the Brocken mountain in Germany.