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?