Wave-Particle Dualtheory

Wave-particle duality.

The behaviour of atoms, light and other particles has fascinated scientists for millennia. It took ages of theorizing before someone came up with experiments with which we could prove how things behaved.

Thomas Young (1773-1829) had proven with this double slit experiment that light was made of waves. (As early as 1678, Dutch physicist Christiaan Huygens proposed that light was a wave). Thomas Young’s experiment was easy to recreate so if scientists were skeptical about his paper, they could recreate it themselves.  Slowly the view changed and after many years another physicist proved that they also act like particles. This physicist was Albert Einstein (1879-1955). Surprisingly they are both correct.

In this blog, I will be keeping it rather short and explain Young’s experiment and the thought behind it. In a later blog post I will explain Einstein’s explanation of the photoelectric effect, explaining why light is also particle.

Young’s Experiment

Thomas Young proved with the double slit experiment that light acts like waves. He made a board with two slits, and behind that there was a light detector.

Interference pattern emerging

When we imagine this experiment with loose particles, like sand, the expected result is two mounds of sand heaping up below the slits. However, this was not what Thomas Young saw on his detector and this result was a shock for everyone.

So in short, if we let light flow through one slit, we get a ‘heap’ of light on the detector. This is what is expected of a particle. If we let light through two slits, we do not see two heaps, but we see what is called an ‘interference pattern’.  This is what we expect with waves. What Thomas Young saw were lines of light, with lines in between, where nothing did hit the detector. This result was however very characteristic for waves. Waves can cancel each other out (when top wave meets bottom wave)  or increase the amplitude of the wave (when top wave meets top wave). Where the wave is canceled out, we see nothing. Where the wave is amplified we can see the landing spot of the light.

Young’s Experiment. With amplified waves in blue, and the negated waves in red.

Again with electrons!

At a later time, scientists tried this again with electrons. Tiny little particles that surround the core of the atom. We know for a fact that electrons have mass, so it has to be a particle. When we set up the double slit again and “pour” a load of electrons through the slits, we would definitely see these two mounds of electrons on the detectors, right?

Well this was not the case. The same interference pattern emerged from the electrons, which means that electrons behave like waves as well! We know they are particles, and yet they behave as waves.

Luckily, scientists often are a clever bunch. They asked themselves; “What if we were to fire one electron at a time through? It can’t interfere with other electrons, and we should see it acting like a particle again.”

It would mean we don’t have the two slits both producing waves that could potentially interfere with each other.

The strange thing is, this is also wrong. When we pour one electron at a time through the slits, we still get this interference pattern. This means that the electron is able to effect itself. The electron goes through both slits at the same time, and influences its own path. Obviously this is very contradicting to what we in everyday objects, but this same effect has been noticed with some atoms and some small molecules. Which means that every particle has their own wave-like nature and their own specific frequencies.

Experimental data from the electron hitting the detection screen. This is with one at a time.

So if there are two slits open and we fire one electron through it, it goes everywhere. It goes through both slits, to the side, in a curve, over the slits, you name it. It will still produce this wavelike interference pattern.

The observer effect.

This is not where things stop getting weirder. When we keep observing the single electron go through a double slit experiment, it doesn’t produce this wave-like result. When we put a measuring device next to one of the two slits, to measure the electron that is getting fired past, the electron only goes through one of the slits. So when observed, the electron acts like a particle! This is called the observer effect.

Why the observer makes the waves collapse is not known. This is what quantum physics is about. To understand these kinds of phenomena. Why the observing of a particle makes it only take one of its millions of possibilities and how this would affect us.

Extra links;

Amazing Professor Brian Cox giving his lecture on the BBC. This is wonderful to watch and in the beginning of part II he performs Young’s experiment!




Physics for scientists and engineers p1298 5th edition Beichner Serway




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