Light is very strange in that it travels as both a wave and a particle. This activity will simplify this concept by exclusively examining light’s particle-like properties.
When two light waves interact with each other, they experience one of two types of interference: destructive or constructive interference. In the Wave Machine experiment, we explained what a crest and a trough were. If two crests or two troughs interact with each other it results in constructive interference. The wave will combine and have a greater amplitude. If a crest and a trough interact, though, a destructive interference will occur and the amplitude will become smaller. In the first half of the experiment, you should have noticed that the two ripples began to “destruct” when they interacted with each other. This reflects what happens when two light waves interact with each other in a destructive manner.
The second half of the experiment demonstrated diffraction. Diffraction is the process by which a beam of light or other system of waves is spread out as a result of passing through a narrow aperture or across an edge. You should have noticed the ripples get bigger as they went through the opening between your wooden slabs. Likewise, when light squeezes through an opening, it tends to spread. For example, when you shine a flashlight at a wall, you may notice that the area the light covers it greater than the area of your flashlight lens. It does this because of diffraction.