When waves move toward the shore, they usually go in a straight line. But what if there’s something in the way, like a big rock sticking out of the water or an opening in a wall like in the image above? Instead of just stopping, some of the waves go around the rock or though the opening and spread out in different directions.

This bending and spreading around obstacle and through opening is called diffraction.

Only obstacles and openings with dimensions comparable to the wavelength of the propagating wave produce diffraction. We discussed wavelength in the article about waves.

When there are two openings next to each other and a wave passes through them, something very interesting happens! A wave passes through both slits and creates two sets of waves on the other side.

What happens when these waves meet? They combine which each other and this is called interference. They combine in a few ways:

Adding together: If the top of one wave meets the top of the other wave, they join up and make a bigger wave. This is called constructive interference.

Cancelling out: But if the top of one wave meets the bottom of the other wave, they can actually cancel each other out, making a smaller wave or even a flat spot! This is called destructive interference.

So, wave interference is what happens when two waves meet: they either join forces to make a bigger wave or cancel each other out to make a smaller one.

If you shine a light at a barrier with two tiny slits cut into it and look at a screen or wall behind the two openings, you see a series of bright and dark stripes. These stripes are called the diffraction pattern, and it happens because the light waves are interfering with each other as described earlier!

This experiment was invented in 1801 by Thomas Young and it is very useful in quantum physics. Read more about its uses in the articles Double-slit experiment and Discovery of the electron as a wave