## Section 5.6: Double-Slit Experiment and Wave-Particle Duality

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Light acts both like a wave and a particle. Electrons act both like particles and waves. Electrons, in the double-slit experiment, create an interference pattern. This means they are behaving like waves. But are they waves?  We know they do not behave like waves when they collide into other objects since, in that case, electrons behave just like particles. Because they act like particles, we tend to think of them as particles (we often have a picture of them as little dots circling the nucleus in planetary-like orbits). But here is an instance of electrons acting like waves.  Restart.

Let's change the experiment slightly. This time, we will put a detector near one of the two slits to determine which slit the electron goes through. After all, the electron must go through one slit or the other, it cannot go through both, can it?

Look at what happens when you do this experiment.  The result is certainly quite different from the original. The interference pattern is gone. It looks as if the signal is clustered directly below each slit. This is exactly what you might expect for a stream of particles to go through one slit or the other. What did we do to get this type of result?  We changed the type of measurement that we did. When we try to measure particle-like attributes, that is what we see. When we try to measure wave-like attributes, that is what we see. This is the heart of wave-particle duality.

We have already encountered particle-like properties of light: we've defined momentum (even though light has no rest mass) as p = h/λ and energy as E = pc = hc/λ. What about the wave properties of the electron (or other particles)?  It turns out that they have a wavelength, called the de Broglie wavelength (Section 5.7) equal to λ = h/p.