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Registered Member #2063
Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352
im just curious to see what would actually happen if you shine a laser in a room where all side are mirrors. would the laser beam keep bouncing off the mirrors forever and as soon as you turn off the laser,the room will dim to complete darkness instead of instant darkness as soon as you turn off the laser
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
If the laser is a perfect emitter and the mirrors are perfect it will get brighter and brighter until you turn off the laser then the brightness will be constant.
In reality it will very quickly reach a state of equlibrium and hold a constant brightness untill you turn off the laser and it will dim very fast. If we assume that the laser will bounce 50 000 000 times a second and there will be a 10% loss for each bounce we get a rough idea of how fast it will dim.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
bwang wrote ... How is the fact that the light source is a laser relevant to the question?
Good point; the laserness is irrelevant. Consider "integrating spheres" used in photometry. If a light source is inside a sphere whose interior walls are painted flat white, the light will be very close to uniform intensity at every point inside the sphere. This is used to measure the total lumens emitted by various light bulbs, fixtures, lasers, etc. without having to scan the far field at all angles.
Back to the original point: as Bjorn said, the time for the light flux to reach equilibrium is a modest multiple of the light transit time across the enclosure. I wonder how the best white paints compare with the best mirrors, in terms of reflecting white light with low loss?
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
bwang wrote ... Well, the "best mirrors" are broadband dielectric mirrors with reflectivities around 99.9%; I doubt paint can beat that.
Interesting - I hadn't known they could be so good over an octave range of wavelengths (at least for normal incidence). I wonder how they perform at large angles of incidence? 60 degrees is the median in terms of solid angle, in the closed room or integrating sphere case. Perhaps the dielectric mirror lining would give a brighter equilibrium and longer decay time, but be less spectrally neutral. Since each ray bounces around until completely absorbed, small differences in loss could be amplified.
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laser in a room full of mirrors?
