Question: Is light a wave or a particle? Or can it be both (Wave-particle duality)? I've read somewhere the photoelectric effect suggests it's a particle but diffraction suggests its a wave. The double slit experiment further confuses things. So which is it? Thank you.

Derek McKay-Bukowski answered on 13 Jun 2011:

(Thanks Tim for passing this one on to me. It makes me not feel so bad about sending all those biology questions back to you!)

So… wave-particle-duality. This is going to be tricky to explain, but let’s give it a try.

Confusion over the wave-particle duality comes about because people assume that something can’t be both. We either think of waves (like ripples in a pond) or particles (like snooker balls). The reality is that everything is both.

Let’s pick a different example. Let’s say that any object can be either lying down or standing up. For example, a piece of paper could be lying on the table. But what happens if it is a book… you know, just a bit thicker. Well, we’d probably still say it was lying down. But what about a really thick book; say a book that was thicker than it was wide or tall? At what point would you say it was no longer lying down, but was standing up? Perhaps you might argue that it comes down to the way you open the book. But does it? So, what about cubes (like some dice from a board game). Are they lying down or standing up? Well.. they are sort of doing both.

Now, going to waves are particles, you could look at these as different things, but it actually makes sense to treat them as different ways of looking at the same property. Some sort of wave-particle-property of matter.

If you take something like radio waves, they are so low in energy that their wave properties are the most obvious. Take something like an atom, then it is the particle-like effects which show up the easiest. However, it was with light, that scientists first realised that there was something here that seemed to be both. And, when they investigated further, it became clear that as you go in either direction, the effect doesn’t stop, it just gets smaller (and thus harder to measure). yes, the radio wave acts like a very weak particle and, yes, the atom has a very small wavelength and can diffract.

Because light was like the cubes in the lying-down/standing-up argument, this problem was first noticed – and solved – here. Early scientists couldn’t accept that it was both and so they thought it was either a particle (or a wave) and thought that scientists who thought otherwise were wrong.

Ultimately, everything (including you and me) are both particles and waves.