Light Box Question

I think thats how a photon torpedo works?

Balance the heat flux produced by the lights with the heat that escapes from the closed box. There will be an equilibrium temperature.

It stabilized as soon as you turned on the "intense light source".

Heat is a different issue. But you didn't ask about that.

A small portion of the light gets absorbed by the non-ideal reflecting surfaces of the box or the atmosphere inside the box. The rest of the light comes out of the opening that the light entered the box. The exception to this explanation is how a laser works. The photons that are correctly phase matched will tunnel through the reflective surface. This is why a laser is coherent.

There is no opening in the box. You can assume a high power Led as the light source. The led is inside the box , so , there is absolutely no light coming out. Let's use a numerical example , the LED generates 100 lumen, 99 % got reflected back on first reflection ,so 99 lumen is added to the 100 lumen originally coming from LEd, then on second reflection, 98 lumen is added, and so on. All there happens while the LED Keep pumping continuously 100 lumen. Other than a small amount loss as heat , this loss is not as high as the lumen being pumped in, so the lumen is getting higher and higher, until ....what?

There are, of course, no perfect reflectors. Even if you make a reflector with a reflectivity of 99.99 percent across some band of wavelengths, it will be semi-transparent, and absorbent, at other wavelengths.

Some of the light will pass through the reflective surface. Eventually (i.e., within a fraction of a second of time) their will be an equilibrium between new photons leaving the LED, photons being re-absorbed by the LED, and photons passing through the 'reflective' surface. At that point you simply have a heat source inside a box. The box will eventually reach some equilibrium surface temperature depending on the wattage being pumped into the box, the material the box is made of, its thermal mass, the surface emissivity, and the thermodynamic conditions surrounding the box - the surrounding temperature, thermal mass, etc.

There are a too many unknowns to say how quickly the box will reach thermal equilibrium with its surroundings and what that temperature will be.

You're looking for a classical mechanics explanation for how a laser works. To understand a laser requires an understanding of quantum mechanics. Depending on the thickness of the walls of your reflective box, laser light will either be released or absorbed by the walls of your box. This will eventually (actually very quickly with a photons velocity) reach an equilibrium where the energy of laser light equals the light energy produced inside the box.

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There really isn't a 'tunneling' effect required for lasers. It's not like the photons need to reach some energy state before they can pass through one of the mirrors. In a laser one mirror is highly reflective (99 percent) but the other is partially transparent (50 percent) so the photons start to leave through the partial reflector right away. 'Phase matching' is not required for the beam to pass through the partial reflector.

The coherence occurs due to the 50 percent that are reflected back which 'stimulate' the release of other photons at the same wavelength and at the same phase as this reflected beam. (The mirrors usually have dielectric coatings tuned for specific reflective properties at the designed wavelength of the laser. In a typical HeNe gas laser one of the mirrors will have 99.9 percent reflectivity at 632.8 nm, while the other mirror has a reflectivity of 50 percent at 632.8 nm. Since 632.8 nm is one of the emission wavelengths of Neon.)

The coherence happens due to the optical resonant cavity formed from the two mirrors. The choice of using a partially transparent mirror on one side selects which end a controlled laser will emit. It is because the resonant cavity supports some frequencies of light and not others that a laser can become a coherent source and not just monotonic. While you are correct that phase matching is not required to pass through the partial mirror, the resonance of the two mirrors makes the light at each mirror surface coherent. So the light that propagates through the partially transparent mirror is coherent.

Very well stated.

My mistake then. It maybe a common mistake then because my "laser theory" link clearly states that a node must exist at each mirror.

You may be confusing the Transverse Excitation Modes (TEM) of the resonant cavity with the frequency sprectrum of the laser. Lasers are monochromatic to the extent that the final transition (that lases) in a three-level or four-level system is a discrete, uncoupled transition, and that Doppler Broadening of the spectrum is not a dominant term in the oscillator strength. TEM can be low order, or higher order, in which the beam does not have a monotonous intensity profile across the width of the beam.

TEM₀₀ looks like a pure Gaussian intensity profile in two dimensions. Other TEM profiles have nodes in them (areas with little to no intensity), and are typically not desirable in high energy laser work, or in most spectroscopy work where defined spatial interaction with the beam is critical to the experiment.

Some lasers are made more monochromatic by the addition of active optic elements in the resonant cavity (such as tunable dye lasers, or the tunable pulsed CO2 laser, etc.)

One of the interesting things about the question posed by the OP in this and another discussion is that the "box" seems to resemble what is known in the optics industry as an "intgrating sphere". Basically this integrating sphere is utilized to bring all of the light into contact with a detector, in order to precisely measure beam power, or energy.

The fact that there is a node at either mirror is simply a statement that a standing wave exists. The same is true for a vibrating string. Or the column of air in a pipe organ pipe.

That's more information, but it doesn't contradict my comment that tunneling is not required. Neither of the links you provided even use the words 'tunnel' or 'tunneling'.

Ok, let me ask you a question then. Are you proposing that the scenario will not involve tunneling at all or that tunneling never happens at either mirror in a laser? My citations describe a laser that has mirrors with different amounts of reflectivity. The OP has suggested a scenario of a nondescript resonant cavity box with unqualified amounts of reflectivity of the surfaces. I brought up tunneling to reinforce the idea that reflections are never perfect. I brought up laser theory because this is the most common scenario I can think of that resembles the given scenario.

I've supported my opinions with scientific documents which are relatively easy to comprehend simplifications of complicated theories known that I believe apply here. No one else has provided anything but personal rhetoric.

I just don't see any need to invoke 'tunneling'. The partially reflective mirror can be treated using classical physics. By the way, in my earlier response (#7) I agreed with, and expanded on, your mentioning of the fact that there are no perfect mirrors.

I've used lasers in my work as an optics engineer. I think a better representation of what the OP describes, internally, is an integrating sphere with a lamp inside: http://en.wikipedia.org/wiki/Integrating_sphere, which I've also used, though most people have never heard of them. An integrating sphere is a device with a highly reflective interior in which photons make multiple reflections before exiting through a port or being absorbed in the wall of the sphere. In the OP's case, he's considering a boxy version of an integrating sphere, with a lamp inside and with no ports. So, in this case, absorption into the walls of the box is all that matters for the long-term equilibrium situation. What we really have, then, is a blackbody radiator http://en.wikipedia.org/wiki/Black-body_radiation, which I alluded to in my #7 post by suggesting that what eventually matters is the overall thermodynamics of the box and its surroundings.

Here's an easy experiment that simulates the OP's question: Take a flashlight, cover the emitting end with one or two layers of aluminum foil, and turn it on. The foil will reflect the light back into the flashlight. Eventually (probably after just a minute or two) the end will get slightly warm. The filament (assuming an incandescent light bulb instead of an LED) reaches a temperature of maybe 2500 Kelvin, but the thermal mass of the flashlight and the loss to the surrounding air never lets the end of the flashlight get more that a few degrees warmer than the surrounding air.

An LED could have an effective color temperature of over 6000 Kelvin, but I doubt that an LED in the flashlight would change the long-term temperature very much, since what matters is how much energy the batteries can pump into the end of the flashlight.

By the way, I looked at the article from the Royal Swedish Academy you asked me to 'decrypt'. I don't see anything in it particularly cryptic, but I downloaded it and converted it to a Word document, so I can spend some more time looking at it later. It deals with using a laser to perform a type of spectroscopy. That may be the type of spectroscopy being used on the latest Mars lander, Curiosity's 'ChemCam', which blasts objects with a laser and analyses the light.

Sorry, but the OP said nothing about a resonant cavity. In fact (s)he said nothing whatsoever about the shape or size of the box; only that no light escapes and that the surfaces are highly reflective. As you indicate, there is no value given for reflectivity.

There is nothing that indicates any lasing action; just emission and reflection of light, with an unspecified amount of absorption at each reflection..

'...In fact (s)he said nothing whatsoever about the shape....'

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'Nothing whatsoever' is going a bit overboard, don't you think?

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While there isn't much to the description provided by the OP, one of the things it does contain (and incidentally, which you repeat) is a term descriptive of shape: box.

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Not much, but certainly more than 'nothing whatsoever'....

"In fact (s)he said nothing whatsoever about the shape or size of the box." I did indeed repeat that term from the OP.

The term 'box' has a long list of definitions; it does not necessarily refer to a rectangular solid, or any other shape. Here, it simply referred to a closed container that prevents light from escaping, and reflects most of the light impinging on it's inner surfaces. The OP may well have had some shape in mind, but made no statement regarding it. In fact, my mental image when I first read the post was of a more-or-less spherical solid polygon with lots of flat facets, but it could be any hollow solid shape.

I reiterate 'nothing whatsoever'.

box/bäks/

From www.merriam-webster.com

Box : a rigid typically rectangular container with or without a cover

I draw your attention to the word typically in the definition. Typically a box is rectangular but it can be any shape. It does not have to be rectangular. It does not have to have a flat base or sides. I agree with dkwarner. There is absolutely no information given by the op regarding the shape and size of the box.

You seem to be suggesting that because this definition of the word 'box' is a description of a container merely typically (vs. without exception) rectangular in shape; the resulting ambiguity negates any communication of shape whatsoever.

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I don't dispute that what the OP provided was woefully insufficient. I don't deny that it is at best ambiguous. However saying that nothing was provided regarding the shape of a box is fallacious, since 'box' in itself provides some description of shape.

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Suggesting that a word communicates nothing merely because there is some variability in possible meanings would leave you with very few usable words, since almost every word has that characteristic to some degree.

For example, if the OP had described a black box, would you insist that nothing whatsoever had been provided concerning the color of the black box, merely because 'black' might possibly describe something other than color (perhaps a flight recorder)?

No thinking your way out of this diversionary thread, so it seems.

It's boxed in.

It won't stabilize. It'll get brighter and brighter ^{and brighter and brighter} _{and brighter and brighter ^{and brighter and brighter .... .. .}}

If the material of the box is not completely rigid, it will vibrate or compress locally under the barrage of photons and become hot. If you continue to pour energy into a closed system, it must get hot one way or another. This problem is a non problem because there are no materials that are rigid and insulate and reflect.

With all above mentioned assumptions the quantity of photons, which could not be absorbed by the reflectors and could not escape from the box, will increase. This process will be continued till photons damage the light source and stop increasing.

Probably not.

I am wondering why this question has resulted in answers that are digressing into the field of laser theory when the answer is much more straight forward.

Don't worry. You'll get used to it.

Yep. And you'll often find that you give an answer, then someone else comes along 17 or 18 responses later and gets a bunch of GAs for essentially repeating what you already said.

GA

Post #2 sort of summed it up first. After that is is all babble, but we like it that way.

Run! Run! Before it explodes!

Anyone else here thinking how this question might be solved with this technique?

If the reflecting surface absorbes 10% of the light incident on it and reflects 90% back then for every unit of time the brightness would increase till till it reaches equlibrium at about 10 times the brightness of the light source. Say, after first unit of time , the light released is 100, the light reflected is 90 and absorbed is 10. At the second unit of time another 100 is released ,out of which 90 reflected + the 90% of the earlier reflected light which is 81 added hence the brightness becomes 171. At the third unit of time simlarly it becomes 244 and so on till it reaches close to 900. If the reflecting surface absorbes only 5% then the brightness would reach just below 20 times of the light source. Since the distance between the reflecting surfaces are too small compared to the velocity of light, the total unit of time taken to reach this level is too small and if the source is switched off the exactly in the same time it will be completly absorbed which is therefore very difficult to detect with human eyes.

Don't know about all that stuff. But when I was working construction building power plants, we would build a box out of plywood line it with tin foil and put in at 100 watt light bulb. It would cook some mighty good steaks and baked potatoes.

It was an incandescent light bulb, no doubt.

It would not work with an LED or fluorescent bulb. 80% of the energy burned by an incandescent bulb is emitted as heat, that is what baked your spuds.

Easy Bake Oven

Just a comment.....

Your light source be it visable or IR is a source of energy.

So not to get very technical, here is kindof a rule of thumb:

If you apply, say ( 1mW) into a perfect insulated container the temperature will become infinite.

So, your power is your light source. And, if your reflective surface is perfect your temp will continue to rise.

Now you have to figure in the heat losses.. How imperfect is your insulated container.

If the box is a nearly perfect reflector and insulator, the heat will build until the light source melts, that will short the supply and the system will slowly cool to ambient temperature.

Energy cannot be destroyed and cannot be created but coverts into one form or the other. In this LED is fed with electrical Energy and this energy is being converted into 90% light + 10% heat( as assumed). As electrical energy continuously pumped into LED which is inside a shielded box from any kind of radiation. Owing to heat generating continuously in the box which once reaches 125C^o .Then LED-Junction will thermally runaway and LED (a Kind of diode) will be come from LED to a simple electrical wire as junction(P-N junction) of Led being heated to 125C⁰ and fuse at supply end of Electrical power open the circuit due to short circuit( from LED or diode to Electric WIRE). So no more light & heat increases in the BOX.

Before creating new theory or an explosion, we will all be safe from any kind of hazard. Thanks to all

Question...... Lets look at it from out side the box for moment.

I mean way out.

Think about all the stars in the universe that have been pumping light all this time,

and this light sources are on average diameter a million mile .

Yes the distance is great, and yes is dark at night and no mirror.

Now think, DARK ENERGY AND OR DARK MATTER ????

Would this be the same.

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Intelligence is memory.

Lets take care our little blue dot. it the only HOME we have

No.

I agree with Anonymous Hero. Why would this be the same? I fail to see any connection. Maybe I'm blind - please enlighten me.

In #6 you said all light is reflected. In that case the luminosity will increase until the radiation pressure from the reflections equal the radiation pressure of the LED. At that point the luminosity will stabilize.

How do you know the light is on?

If you can see it then it can escape. If you can see it, you will affect it.

Faith

It only exists in the domain of potentia. :)

In the black arts part of my education (actually, a refresher course when I needed the EIT in my forties) known as Thermodynamics, the prof. asked:

If there is a refrigerator with the door open, in a perfectly sealed and insulated room with only an electrical supply for the refrigerator, what would happen to the temperature in the room. (This has been on here before)

It is basically the same problem, with the same answer, up to a point, the room would get hotter because energy is going in and not coming out (shades of the Ultraviolet Catastrophe). The limit, as SG said, is when the luminosity of reflection equals the LED. For the refrigerator, I suppose the air would eventually reach the temperature of the compressed gas and cooling would stop. would either one explode? Beyond my knowledge or conjecture.

I love that puzzle because most people say it gets colder and are corrected.

However, in reality, they are right, but only for a period of time because there is a large sink of cold air and mass in the refrigerator (particularly if you had put that giraffe in there from the previous puzzle) that will overcome any waste heat generated by the compressor.

In the end it will be hotter, but it will take some time.

Thank you all for your comments. A number of the comments dwelve on the thermal equilibrium, the lighting level within the box was what I was interested in

Further to my post #6 and #30 from KV Gopal , interestingly I realised that the lighting flux level is actually a summation of GP series, with the GP ratio being the reflective ratio.

If you sum it to infinity , the sum is = A/(1-r) , where A is the first term, A is 100 lumen in my example.

So, if the reflective ratio is 0.99, the steady state brightness level is simply 100 /(1-0.99)= 10,000

Also, if you deduce from principle, the steady state will be reached when the brightness reach such a level that input lumen (=100) equal the loss lumen (absorped) .

If loss is 1% , steady state flux level Q will be given by Q x 0.01 = 100 , Q=10,000

the result is same as summation to infinity of the GP series !