# Detecting a light ray without disturbing it If I have a light ray trapped between two very reflective surfaces such that the light ray doesn't dissipate, how exactly can I know that there IS a light ray there, without disturbing it? As far as I understand, for me to actually observe the light ray with my eyes means the light ray coming out of this loop towards my eyes, escaping the loop. An easy way to test it I suppose is to stick my hand in it and see if my hand lights up, but this dissipates the ray doesn't it? When I remove my hand, the ray shouldn't be there any more. (as far as I understand). Is there any test I can perform to detect this ray without disturbing it? I was thinking since it is an oscillating electric and magnetic field, there should be a test perhaps?

• You can measure the force between the two mirrors. Without light the force should be zero, with light there will be a small force pushing the two mirrors apart. – CuriousOne Jan 4 '15 at 0:33
• @CuriousOne one thats right. Make it an answer! What the OP is looking for is radiation pressure – Constandinos Damalas Jan 4 '15 at 1:01

You can measure the force between the two mirrors. Without light the force should be zero, with light there will be a small force pushing the two mirrors apart. PhotonicBoom is correct, light exerts radiation pressure on the mirrors. There is a general principle at work here: physical systems exert forces which allow them to lower their total energy. The light that is trapped between the two mirrors has a certain amount of energy. It exerts a force $F_{radiation}$ on both mirrors. If we allow the two mirrors to move apart by a small distance $ds$, that force produces a small amount of work $dW=F_{radiation}ds$ and the energy of the light between the mirrors has to decrease by that amount because of energy conservation. In other words, the light acts very much like a gas would act in a cylinder. In case of a gas the gas molecules would be getting slower and colder. Light can't get slower, but it can "cool down" by shifting its frequency lower and it can decrease in energy density. So as we are pulling the mirrors apart, the light would slightly redshift. If, on the other hand, we were to push the mirrors together, it would gain energy density and it would blueshift.