Shadows and Ray Diagrams Shadow: the dark region (absence of light) behind an opaque object being illuminated on one side. Umbra: part of the shadow in which ALL light rays from the light source are blocked. Penumbra: part of the shadow in which there is partial shadow (applies to broad light sources). Ray diagram: Show only the light rays that pass the edges of object that is illuminated.

1. Shadows: Point Sources A point source is such a small light source that it can be considered a point. All light rays depart from that point. Shadow PS Umbra Screen Illuminated Area

Now look in the direction of the source with the holes in the screen. A B PS Screen An observer at A (illuminated area) sees the PS. An observer at B (umbra) does NOT see the PS.

2. Shadows: Broad Sources Screen Penumbra Shadow Broad source Umbra Penumbra Illuminated Area

Now look in the direction of the source with the holes in the screen. With a broad (larger) source (eg. the projector light), the shadow can be seen but its shadow edges aren’t as sharp in a point source. There is a gradual transition from darkness to light. Now look in the direction of the source with the holes in the screen. A Broad source B An observer at A (illuminated area) sees the source is fully acknowlged. An observer at B (in the shadow – umbra) does not see the source.

Broad source C An observer at C (penumbra) sees only part of the source (its bottom half).

Consider the case of a broad source and two objects. Umbra Umbra Penumbra

Study Questions: What property of light is demonstrated by the formation of shadows? Why is it not necessary to draw more than a few light rays when drawing ray diagrams for shadows? What are some practical ways in which we use shadows?