1. 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.

2. 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

3. 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.

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

5. 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.

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

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

8. 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?