• Question: I see that you work with light and stuff! I recently had a few questions about photons-which are what make up light, I think? So, you'd be the perfect person to ask! :D Photons act like both waves and particles, so, a previous question I had was could you make a photon simply behave as a particle and, if you could what type of state would it hold? Also, could photons fill up a room, like gas, so for instance if you had a sealed room with a crack at the bottom and then you released photons through the crack, would they fill the room up, or just the area around the crack? If they wouldn't, would they if they acted like a particle?

    Asked by DrProfessorKit to Fi on 6 Jan 2015.
    • Photo: Fiona Dickinson

      Fiona Dickinson answered on 6 Jan 2015:

      Let me start off by saying ‘light is weird’ if you don’t end up thinking light is weird go back and read again!

      Light can be thought of as either a particle or a wave, when light has particle like properties it is called a photon. There have been a number of very important scientists who have tried to understand light, from Newton (who found white light could be split into a spectrum using a prism) who said light was a wave, to Einstein (who found you need light of enough energy in order to do certain reactions) who said light was a particle, to Richard Feynman (who is my scientific hero) who said it was neither a particle or a wave, but can be described as a particle or a wave in most circumstances (is it weird yet?)

      Now photons are a great way to think about light for when it interacts with matter (and it the picture I normally use in my head because it works best for what I do), whenever light interacts with matter it can either a) do nothing b) be scattered in a different direction c) be absorbed by the molecule or d) in some cases make the molecule give out some light.

      The experiment you described ‘filling up a room’ and letting some out of a crack, is actually remarkably similar to the experiment which started physicists on the path to quantum mechanics, where they heated a cavity (a metal box with a sphere cut inside it and a little hole to the outside world) and looked at the light coming out.

      So why did I start talking about photons, well Max Planck (who won a Nobel prize for this) found out that the light coming out of the cavity had certain properties (photons). The light coming out of the little hole only travels in a straight line (wave) (the Greeks knew this), so it will make a little cone, but it will not ‘fill the room’ as a gas does, however I said about scattering (b) or (d) giving out some light so any surface the light hits the light will spread a little.

      Eventually all of the light will be absorbed by matter (photon), but for a little while it will bounce around the room and give it a little bit of illumination.

      So how do I know this has to happen and the wall doesn’t absorbed all the light? Well if I go into the dark room with the cavity and the light shining from it if I can see the dot on the wall where the light hits it some of the light has to have bounced off the wall and into my eye.

      A gas fills a room and when it collides with a wall you will always have a gas molecule so eventually the room is fall of gas colliding with the walls and other gas molecules.

      Light can be absorbed by the walls and so eventually we run out of photons. As I see I am ‘using up’ photons of light in my eye.

      So what have we learned about light, it is neither a particle nor a wave, but sometimes it is convenient to think about it as either a particle or a wave. It can’t ‘fill’ a room like a gas could, but in any room where we can see the room must be filled with photons. Light is difficult to understand, we can’t predict if it will behave as a particle or a wave, even when we have only one photon (one particle) we still can’t predict what it will do. We just have to find the best explanation we can it works (most of the time).

      I really hope this helped…