question about light

If I put two mirrors at each end of a sealed tube and shine a flashlight inside, why won’t inside of the tube stay lit until it’s opened? Would this change if the entire inside of the tube were mirrored?

[assuming that both mirrors reflect perfectly and do not absorb any light, which they do in real life, not to mention air resistance]How exactly do you shine the light into the sealed tube? you have to shine it directly down on one mirror and then put the second mirror on before the light reflects back. This is understandably hard right?
Okay lets assume that you are able to do this and the light is reflecting back and forth between the mirrors. The light would continue to reflect back and forth, but you would not be able to see it [eg, through a hole in the side, or if the tube is made of glass], as the light is only going in a straight line back and forth, and if you are seeing it, then light is being lost from the tube, and will therefore quickly run out. When you open the tube, you would only see a really quick flash of light, probably not visible to the human eye. Maybe if you made a really long tube…

This would be possible only if the interior was perfectly reflective. But it’s not. Flaws in the mirror surface exist, and are constantly absorbing the photons. And at the speed of light, every photon will run into a flaw fairly quickly.

Even if you had such a perfect interior, you’d never be able to see inside to observe the effect. Putting a monitor in would introduce flaws (the optical receptor to see the photons for one).

If you had such a tube you could add photons to it by opening a small hole in one end squirting a photon through it, and then closing it before the light has a chance to reflect back out. Light only moves at ~9 inches a nanosecond so you’d have plenty of time. If you repeated the process at the right intervals, you could build up an arbitrarily large amount of electromagnetic energy inside the tube.

What if you were inside a mirror lined room, and you turned out the light? For how long would the room stay lit before the imperfections in the mirror “absorbed” the light?

It’s more like a foot a nanosecond, Squink.

algernon, if you’re in the room, then it’s not the imperfections in the mirrors you need to worry about, it’s yourself. At a rough estimate, a light ray might cross the room a dozen times or so before hitting you, so maybe a microsecond.

As Nukeman said, you do not see the light that is going back and forth between the mirrors. You see the light that is being reflected away from the mirrors by dust, or the walls of the container. Even if there is no dust, there are particles of air which scatter the light and some is lost on every bounce. So the beam inside the container wouldn’t last very long.

You can DIRECTLY OBSERVE why it won’t work. Get two big
mirrors, face them towards each other, then stick your head
between them. See the “infinite tunnel?” See how it fades out
into dark green oblivion? That’s the light being absorbed by the
greenish glass and by the metal mirror too.

If you get some expensive “front surface mirrors” where the
silvery coating is on the surface, the “infinite tunnel” looks
different. It goes deeper, and it fades out into grey-black
instead of green. It’s because the metal absorbs 5% to 10% of
light on each bounce. If you want to store light for one second,
then the “tunnel” must be 186,000 miles deep without fading out
to black.

If you use normal everyday 90% reflective mirrors, the light will
be almost entirely absorbed after a hundred bounces. Even if
you used exotic multilayer laser mirrors, the light would still be
absorbed after a couple of thousand bounces. And the light
bounces hundreds of millions of times a second for a box-sized
chamber, so it will die out in a tiny fraction of a second.
number of bounces remaining light
0 100%
10 39% 90% mirror
20 14%
30 5%
Now if you had mirrors which were 100% reflective (and not
99.999999999999%!!!), then you could store light as you say.

Hey, what if you had a special mirror which AMPLIFIED the light?
So that it got brighter and brighter on every bounce? In that
case the depths of that “infinite tunnel” would look bright, and at
the very end of the tunnel you’d see an intense blazing star-like
point. That my friend is a Laser, and it’s producing parallel light
and burning a hole in your retina.

Something akin to this was sold as a novelty item in the seventies kinda like lavalamps and strobe lights. I think they were called “Infinity mirrors” or something. It was a regular mirror and a two-way mirror in parallel with a circle of lights inside so you got repeating images of the lights shrinking to infinity.

I need a pair of those special mirrors to power my perpetual motion machine. Please tell me where I can obtain a pair of them.

Thanks in advance.

Virtually yours,

DrMatrix

As many people have noted, you lose light to absorption in the mirrors, cattering, etc. I don’t think that anyone has mentioned “walkoff”. Unless your mirrors are perfectly aligned, and the light shines directly perpendicular to the surface, it’s going to rattle back and forth an eventually run into the wall. A better way is to use slightly curved mirrors, placed an ppropriate distance apart, so that the beam is constantly re-directed toward the center.

Another cever dodge is to put a source of optical gain in the cavity that makes up for the light lost to scattering, and maybe adds even a little more. Eventually you get a powerful beam of light running back and forth in the resonator.

Congratulations! You’ve built a laser!

Isn’t fiber optic cable a practical attempt at something like this? Shine a light down a reflective tube, encoding information in it? It would also a practical demonstration why the perpectual reflection doesn’t work since fiberoptics don’t work over very large distances without repeaters.

Yeah. The “total internal reflection” phenomenon in fiber optics comes very close to being a 100% reflector. Unfortunately it only works when you go from an optically dense medium (glass, plastic, water, etc.) into a less dense medium. So now we have a 100% reflector, but it becomes impossible to put empty space between the perfect “mirrors!”
PS, if you face three large mirrors towards each other with edges touching, then stand inside the cavity, you’ll seem to be standing on the surface of a planet (or of a bowl, if the alignment is wrong in the other direction.) Many science museums use this as an exhibit, but they use cheap crappy plastic mirrors, and it destroys the effect. The Exploratorium museum in San Francisco has a good one which uses actual glass mirrors. It’s called “duck inside kaleidoscope.” Very eerie. But I wish they had a way to extend the mirrors all the way to the floor, so you’d appear to be standing upon an infinite plane (or upon a large rug-covered asteroid.)

Mayby abit of a highjack but,
Does the frequency of light change over distance?
I mean like it slowly turns form light to radio waves, assuming if in vacuum and gravity does not act on it?

In vacuum and ignoring gravity, light will keep the same frequency forever. In real life, light in the Universe scatters off of dust, etc, and the frequency gets lowered (eg. the Compton shift). The expansion of the Universe also shifts the frequency, from whatever justa fter the Big Bang to current microwave/radio frequncies. (The frequency was as high as you like if you pick a time close enough to the BB.)

The expansion of the Universe is described by General Relativity, so it is essentially an effect of “gravity”.