Rectilinear propagation of light. Shadow and penumbra
In a homogeneous transparent medium light travels in a straight line. We know it because a sunbeam through a hole appears straight in dusty air, and because light passes through holes in several cards only if the holes lie on one straight line. A straight line showing the direction of light is called a ray. Behind an opaque body there is a region where light does not reach — the shadow. If the source is a point source (very small), only a shadow with a sharp edge forms. If the source is larger, a partly lit penumbra appears around the shadow. The nearer the body is to the source, the larger its shadow. Shadows are used in sundials to tell time; in the morning and evening the shadow is long, at noon the shortest. Media by transparency: transparent (glass, clean water, air), translucent (frosted glass, paper) and opaque (brick, metal). To see something, light must reach our eyes along a straight line.
Experiment: in a dark room make a shadow on the wall with a torch and a hand (or toy): move the object closer to and farther from the torch and watch the size; then use two torches to see penumbra. Safety: do not shine the torch into eyes; use an electric torch rather than a candle.