• Question: If you had a box that was completely reflective on the inside, could you trap light, so none is absorbed it just bounces around forever and would it help anyone?

    Asked by moseleyj to Andrew, Daniel, Hayley, Natalia, Peta on 15 Nov 2011.
    • Photo: Daniel Scully

      Daniel Scully answered on 15 Nov 2011:

      Theorists use situations like this to simplify their calculations – because real life is too complicated to do calculations.

      But practically it would be impossible to create a material which was completely reflective – the light would always loose energy when being reflected.

      Even if you could make such a box, it wouldn’t be of any use to anyone… except theorists who want to check their calculations.

    • Photo: Peta Foster

      Peta Foster answered on 15 Nov 2011:

      Great question!

      I like it so much because i have asked it myself and it is basis of how every continuous laser works! I am sure you will have seen lasers in some form, at a laser show, or maybe you have a laser pointer… great for making a spot for cats to chase! 😛 Well the laser is essentially exactly what you have described with one exception… you allow a tiny amount to escape but only in one direction.

      The box that you describe, we call a ‘laser cavity’ and we put something in the middle of this which we called the ‘lasing medium’. In my laser this is a piece of pink crystal called Titanium sapphire. We then flash the crystal with bright light and it excites the titanium atoms. These then release their light in all directions… but … the direction that has the box ends (which are mirrors) reflect the light back towards the crystal…

      Now comes the clever bit…

      When one of the light packets ‘photons’ travels back through our crystal it stimulates the other atoms it travels by the release their energy in perfect sync with the stimulating photon.

      You can imagine this a bit like a conga line dancer dancer dancing through a town and encouraging people to join… when they do, they join in in step… they all hop at the same time and kick at the same time… as do the simulated photons… they are electromagnetic waves which, because they are stimulated, all rise at the same time and fall at the same time.

      So now you have more photons going in the same way and in the same direction out of the crystal… they bounce of the mirror at the other side and are returned back through the crystal to produce even more… you could do this with really good mirror and and have light in a cavity for quite a while… but it the real world a little bit gets absorbed with each bounce and so you need to keep creating more by pumping the crystal atoms up and letting the laser light keep increasing.. then you make one of your mirrors only 99% reflecting and out comes your laser beam 😀

      As for it’s usefulness…. i may be bias but they are fantastic and they are everywhere. At the checkout at the shops, in your computer, Cd player… they are used for cutting, welding, medical operations, diagnosing the cloud conditions, aligning anything you might want to align and so much more… they are even what make the honeycomb of a crunchie smoothed ready for the chocolate to go on!

      Laser light is awesome! 😀

    • Photo: Andrew Cairns

      Andrew Cairns answered on 15 Nov 2011:

      The others are much more qualified than me to answer this, so listen to them! The main issue I saw is with the ‘completely reflective’ surface: that is pretty much impossible and makes this a theoretical issue. It is similar to a classic interview question:

      “If I have a fridge switched on with the door open in a completely insulated room, will the temperature go up or down in the room after a week?”

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