# How much light can be captured and stored in a fixed volume?

## Main Question or Discussion Point

If I have a box lined with mirrors and I shine some light in the box and then close the lid, will the light stay in there? If I turn the lights off in the room and then open the box will light come out?

If it is possible to store light, how much light could you stuff into a fixed space? Is there something equivalent to "pressure" for light?

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negitron
Light cannot be stored in this manner. Even the very best mirrors we can make only reflect about 99% of the incident light--th rest is absorbed by the mirror--and even if we could make one that reflects 99.9999%, it would all be gone after a few thousand reflections. Even in a room-sized mirrored box, that's mere nanoseconds.

sjp9220
I doubt an actual box with mirrors would trap light for a significant lifetime, but as an undergrad, I did a research project with glass microspheres. We trapped light within spheres via total internal reflection near the surface of the sphere. I'll look up the mean free lifetime for you.

Actually, I don't have that data on this computer, but I remember it was much less than a second. This would have been the average lifetime for a single photon before tunneling out, being absorbed, or being scattered.

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Black hole.

Lok
Yes light exerts pressure. Radiation pressure as it is called.

The light in the box will push the walls of the box in a manner "similar" to a gas although the force is very small.

Nabeshin
Yes light exerts pressure. Radiation pressure as it is called.

The light in the box will push the walls of the box in a manner "similar" to a gas although the force is very small.
Right, since photons carry momentum they can produce a force and hence a pressure. Their momentum is given by:
$$p=\frac{h}{\lambda}$$
So, assuming we have perfect mirrors, $$\Delta p$$ would be twice the incident momentum. To change this into a force, though, you would need to know $$\frac{dp}{dt}$$, which has to do something with the photon being absorbed and re emitted, I would imagine.

Staff Emeritus
2019 Award
it would all be gone after a few thousand reflections. Even in a room-sized mirrored box, that's mere nanoseconds.
I think you mean microseconds.

we think that it is 101% possible because if we make our inlet hole for the light, a minima for every interferece that happens between the reflected or diffracted rays,we can minimize our losses .....

please suggest us a geometry and material for which this project will be fruitful to our coming generation........

we are thinking about the concept of diamond or microspheres.....

basically,we want to compress the energy like gases.......

As a child on vacation in Florida, I purchased a can of Florida Sunshine, opened the can in my closet back in Michigan, guess what? no light. I cried, learned a valuable lesson about hucksters and physics. There are no perfect reflectors, light energy is eventually absorbed over time. Only real possibility is storage in black hole where gravity simply alters path of light so it can't escape however that is not to say it won't eventually be absorbed by contact with material that has been captured by the black hole. My guess is that if there were no more material left to fall into a black hole it would become starved for energy and contract into the a very small volume of super heated and pressurized hydrogen, the basic building block of all other matter, when a critical concentration is reached the atomic structure of the hydrogen will be converted to pure energy at tremendous heat. At that moment all matter has been converted to energy and momentarily matter ceases to exist and gravity as a consequence ceases to exist, being itself a property of matter, this results in another big bang and the birth of a new universe of expanding energy . This expansion of energy as it cools gives rise to creation of matter as superheated hydrogen gas as it cools through expansion. This gives rise to other elements as the hydrogen subatomic particles interact with the expanding energy field. By the way, this has already happened along those lines once before in the distant past and if you will only look in the mirror you will see the proof looking back at you.

Two concepts come to mind.

2. A solid state laser with a doped crystal laser rod and two end mirrors that form a feedback cavity.

The total power per unit surface area emitted (also known as the radiant exitance) by a black body radiator is given by the Stefan–Boltzmann law.
E = sigma * T^4
where
sigma = 5.6704E-8 J/(s*m^2*K^4) = Stefan–Boltzmann constant
T = temperature of the black body radiator.

If the black body radiator is spherical then the total power emitted is
P = A*epsilon*sigma*T^4 = pi*epsilon*sigma*d^2*T^4
where
epsilon = emissivity of the black body = 1 for a perfect black body
A = pi*d^2 = the outer surface area of the black body
d = diameter of the black body.

Thus the power per unit volume (V = pi*d^3/6) is
P/V = 6*epsilon*sigma*T^4/d.

One could also estimate the power density in a laser cavity. Maybe I’ll do that later.

f95toli
Gold Member
If it is possible to store light, how much light could you stuff into a fixed space? Is there something equivalent to "pressure" for light?
If it is possible to store light, how much light could you stuff into a fixed space? Is there something equivalent to "pressure" for light?
This is how a microwave cavity works(microwaves are just low frequency light, remember), they are used in all sorts of applications (mainly filters). The best cavities have quality factors of a few billions which give damping times of about a 100 milliseconds.
see
http://www.cqed.org/spip.php?article167

The same thing can be done for visible lights (although then the cavities are usually called etalons), but then the damping times is much lower.

I don't think there is a clear limit when it comes to how much light you could store. However, as the energy density in the cavity increases the pressure from the radiation increases which in turn deform the cavity, this shifts the cavity frequency (which means that the light already in there leaks out) and lowers the quality factor. Hence, how much light you can put in there depends on how mechanically stable the cavity is.

I should perhaps point out that this is not an "academic" problem, there are plenty of applications where the radiation pressure limits the performance of cavities; a high quality factor means high frequency selectivity which in turn means that even tiny changes in the shape of the cavity can be detected.