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Wave Interference

    I've heard scientists refer to "bizarre quantum behavior." What do they mean? When we look at matter on an extremely tiny scale, we see surprising behavior we call "quantum." It strikes us as bizarre because it is so different from our everyday experience. A famous physicist named Richard Feynman felt that all the strangeness of quantum behavior can be seen in one simple and elegant experiment called the "Two Slit Experiment."

    Oh, we got to do that in science class. That's where you shine a laser through two tiny slits and it makes a pattern on the wall. What's so weird about that?

    Let's start with what you remember about that experiment. Well, we talked about how light travels in waves, and waves can "cancel each other out" if the high point of one meets the low point of another.

    Adding Waves

    The bottom wave on the left is the sum of the two waves above it.

    Try changing the amplitude and frequency of the top two waves by dragging the gray ball around.

    You can also slide the whole wave back and forth by clicking and dragging anywhere except on the gray ball.

    The graph at the bottom right shows the relation between the two waves. You can modify the waves by dragging the balls around on the graph.

    Right, or they can amplify each other if two high points (crests) or two low points (troughs) happen to meet.

    And when the light passes through each slit, it spreads out like ripples on a pond, and when the ripples from one slit meet the ripples from another slit, they "amplify" and "cancel" each other.

    Your pond analogy is a good one, because the same thing would happen on the surface of water, say if there were two holes in a breakwater that waves could get through. Let's take a peek at what that would look like. The image below shows two radiating waves interfering with each other. The bright areas are peaks, and the dark areas are troughs.

Interfering Waves

Dark Areas = Troughs (valleys)
Light Areas = Crests (peaks)

You can also see these waves moving as they interfere with each other.