[Science] Wave/Particle Duality

[Acco]

Author: chris_uk_83
Original Thread: Wave/Particle Duality

Wave/Particle Duality

We all know that light is a wave right? We’ve all done the experiments in high school physics that show that a beam of light will spread out when you pass it through a narrow slit. This effect is known as diffraction and is a property that only a wave can exhibit.

Another way we can show that light is a wave is through interference. This is where we point two coherent beams of light at the same target and we see a series of light and dark fringes. Coherent means that the wavelength of the two beams is exactly the same (you can only do this using lasers, since light bulbs give out a large range of different wavelengths). The following diagrams show examples of constructive and destructive interference


Constructive interference occurs because peaks in one wave hit the target at the same time as peaks in the other wave.
Destructive interference occurs because peaks in one wave hit the target at the same time as troughs in the other wave.

By thinking about this for a while, you can see that at different distances from the source (or different locations on a target screen), the amount and type of interference will change. This means you get what is known as an interference pattern on the screen. It looks a bit like this:



The only way this pattern can occur is if light is a wave. If you’re still not convinced have a look at this lovely Wikipedia article until you are.

However, light can also be thought of as a particle because of something called the Photoelectric effect. In the photoelectric effect, light is shone onto a metal surface causing electrons to jump off the surface. This is because light gives energy to the electrons in the atoms that make up the metal, allowing them to leave the captivity of their host atom. If light were a wave, then increasing the intensity (brightness) of a beam of light would increase the energy given to each electron, and would cause more tightly bound electrons to jump off the metal too. As it turns out, experimentally this doesn’t happen. The only way to increase the energy of the light beam is to increase the energy of the individual photons that make it up. A photon is effectively a particle of light. You increase the energy of a photon by decreasing the wavelength (or increasing the frequency) of the light beam (i.e. going from visible light to ultraviolet).

The only way the photoelectric effect can happen (and it is proven experimentally) is if light is a particle. Still not convinced? Look at this article to convince yourself.

So wave is both a wave and a particle depending on the particular circumstances of the experiment your carrying out. But what if we force it to behave like both at the same time? Let’s see.
Enter Young’s Double Slit experiment. This is similar to the way you show interference that we discussed above. Send a beam of coherent photons (a laser beam) through two closely spaced slits to produce two coherent beams of photons, then observe the interference pattern created on the screen. What happens though if you send the photons through one at a time (and this can actually be done experimentally)? Well, each photon has an equal chance of going through either slit, but common sense dictates that it must choose a slit i.e. it can’t go through both. This should stop any interference, because how can a single photon interfere with itself? Therefore no interference pattern should build up on the screen.

The funny thing is that when you do this, an interference pattern still builds up on the screen. The only way this can happen is if the single particle goes through both slits at the same time! What!? ‘Right’, you think ‘I’ll find out which slit it’s going through’, so you attach a detector to the slits. Now you can measure which slit the photon goes through, to see if it really is going through both.

What happens when you do this is that you find that each photon goes through only one slit, and roughly half of the photons go through each slit. What you also find is that the interference pattern on your screen does not happen any more; the photon is now behaving exclusively like a particle and all you did was look at it!

This shows that a photon can be a particle or a wave, and indeed will be both if you don’t pay too much attention to it. As soon as you start trying to look too closely though it takes on one behaviour over the other. Confused yet? You should be!

What gets even weirder is when you start getting particles like electrons and protons to behave like waves! Which they do! You can create an interference pattern on a screen by firing a beam of electrons at the double slit described above! :hmmm: