Naked Science Forum
General Science => General Science => Topic started by: ukmicky on 09/02/2008 18:03:18
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How bright can light get. is their a limit to how bright something can be.
If somehow with the aid of Neil we found a way to focus the light from several trillion trillion torches out of one single torch what would the effect be .is there a point where their would be no noticeable difference [:)]
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There really isn't a straight answer to this one; it depends what you mean by the question.
A laser can produce an intense spot of light. This would be light with a very narrow waveband. The size of the spot would relate to the actual wavelength of the light, too. The spot would have to have dimensions of the order of the wavelength of the light, used. For a red laser light, the size of the central area of the spot would be around 600nm (nanometres) and for blue light, about 450nm.
Diffraction prevents the spot from being smaller. If the light were focused in empty space, there is no limit to how bright / intense it could be but there is a limit in a medium (a gas, for instance) when the electric field strength is high enough to ionise the atoms and a spark would occur. Lasers can produce many Megawatts of power in the form of brief pulses; it depends on how much money you want to spend. . . .
Of course, if the spot were directed onto an object, it would ionise the surface blam! James Bond eat your heart out (or possibly another organ).
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If we ignore lasers you can't focus light to make it any brighter than where it started. And because as something gets hotter it starts to radiate it is impossible to focus light onto a point and make that point hotter than the light source.
Because lasers work differently the rules are different.
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If we ignore lasers you can't focus light to make it any brighter than where it started. And because as something gets hotter it starts to radiate it is impossible to focus light onto a point and make that point hotter than the light source.
Can you explain the reason? I don't know how to prove it.
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Empty space wouldn't have a temperature! The EM vectors would add together without limit because it's a linear medium.
The temperature of a radiating body governs the spectrum. If light with this spectrum arrives at another absorbing object then equilibrium will be reached when the rate of radiation is the same as the rate of absorption of each spectral component. Perhaps this means that you can't get any more 'blue end' radiated than the 'blue proportion of the received radiation; i.e. the temperature of the receiving object can't exceed the temperature of the source. This relates more to temperature than to intensity, though. But you could never get more energy absorbed than was transmitted in the first place.
Because lasers work differently the rules are different.
The in-phase behaviour of the photons is different but the optics of the reflecting mirrors (diffraction) is still the limiting factor in the beamwidth. Lasers are less fundamentally different than people imagine. There are many coherent sources of em radiation which are not lasers! A satellite transmitting antenna is just one example.
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Can you explain the reason? I don't know how to prove it.
I am not sure I can prove it off the top of my head without resorting to thermodynamics, If you could could focus the heat radiation from a large flat thing to a point, and make this point very hot, you could then run a heat engine from this point back to the large cool flat thing - essentially a perpetual motion machine.
I think optically it is to do with how small you can make an image of an object compared to how much light you are collecting...
I am had waving now but isn't on extreme of focusing an ovoid with your source at one focus and another object at the other?
[diagram=316_0]
Now all the light emitted from the source will hit the second object, but the image will be the same size as the source and because it will start emitting too, when it gets to the same temperature as the source it will be loosing exactly the same amount of heat as it is gaining.
I don't think it is possible to focus all the light from the source down any smaller than it's own size, but my optics isn't good enough to proove this I am afraid.
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I don't think it is possible to focus all the light from the source down any smaller than it's own size, but my optics isn't good enough to proove this I am afraid.
Dave - what about using a magnifying glass to concentrate the light from the sun to set light to paper? Surely, that forms an image that is smaller than the source.
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Yes but it only concentrates a tiny proportion of the light leaving the sun and I don't think you can concentrate it to produce a higher power per unit area than is leaving the sun.
You can heat a piece of paper to a tiny proportion of the temperature of the surface of the sun and still set light to it!
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Sorry, Dave, I misinterpreted what you'd said. I thought you meant concentrating all the light that was coming your way.
I really should read threads properly before I poke my nose in [:I]
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Can you explain the reason? I don't know how to prove it.
I am not sure I can prove it off the top of my head without resorting to thermodynamics, If you could could focus the heat radiation from a large flat thing to a point, and make this point very hot, you could then run a heat engine from this point back to the large cool flat thing - essentially a perpetual motion machine.
I think optically it is to do with how small you can make an image of an object compared to how much light you are collecting...
I am had waving now but isn't on extreme of focusing an ovoid with your source at one focus and another object at the other?
[diagram=316_0]
Now all the light emitted from the source will hit the second object, but the image will be the same size as the source and because it will start emitting too, when it gets to the same temperature as the source it will be loosing exactly the same amount of heat as it is gaining.
I don't think it is possible to focus all the light from the source down any smaller than it's own size, but my optics isn't good enough to proove this I am afraid.
Ok, thank you Dave, you were quite convincing.
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Ok, thank you Dave, you were quite convincing
He's becomiing quite the artist as well.
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Thermodynamics has got to be the answer. You can't get owt for nowt, as they say. But I haven't got it straight yet.
In an ellipsoid mirror, objects at the two foci must reach equilibrium (i.e. each one losing the same energy that it was gaining), if no energy is lost on reflection in the walls of the mirror.
But, if they have different areas, then the smaller object would have to be at a higher temperature than the larger one, for this to happen. A small bright white object and a large red hot object - sounds like nonsense.
A bit of a paradox, in fact.
Lightarrow, please resolve this for me (or any other plucky young person).
On second thoughts, I think it might be ok; both objects would be radiating and absorbing the same amount of energy but the big object might have more internal energy - because of its thermal capacity.
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BUT, the original question does not actually involve equilibrium. Does this make things different? take the situation at switch - on.
It would seem that the Watts per square cm transmitted from a large source would be less than the Watts per square cm received by a small source, so the image would be 'brighter' than the object.
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Thermodynamics has got to be the answer. You can't get owt for nowt, as they say. But I haven't got it straight yet.
In an ellipsoid mirror, objects at the two foci must reach equilibrium (i.e. each one losing the same energy that it was gaining), if no energy is lost on reflection in the walls of the mirror.
But, if they have different areas, then the smaller object would have to be at a higher temperature than the larger one, for this to happen. A small bright white object and a large red hot object - sounds like nonsense.
That's an aprroximation, however, since the fact light from F1 goes in F2 is true only for the points F1 and F2, not for extended (that is, not-pointlike) objects. (But I'm sure you already knew it and you just wanted to see if I knew! [;)])A bit of a paradox, in fact.
Lightarrow, please resolve this for me (or any other plucky young person).
On second thoughts, I think it might be ok; both objects would be radiating and absorbing the same amount of energy but the big object might have more internal energy - because of its thermal capacity.
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If you draw a sphere around your 'pointlike object' then the illumination of one sphere (watts per square cm) will only depend on the surface area of the sphere. N'est pas?
I am still struggling with this one because of the original question. The ellipsoid model seems the best way to approach a solution the limitations of the optics would not really affect the result as long as the ellipsoid were big enough.
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If you draw a sphere around your 'pointlike object' then the illumination of one sphere (watts per square cm) will only depend on the surface area of the sphere. N'est pas?
But if you substitute the point source with an emitting sphere whose emission intensity per unit area equals the illumination it would have with the point source in its centre (if this is what you mean), it's not the same since the sphere surface emits also in the direction perpendicular to the sphere's radius, and those light beams don't all focus on the other foci.
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I'm sure this has been tried before.
If you were to point a telescope at the sun, it's going to focus the light to a point where it would be dangerous to even go near it...isn't it?
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it's not the same since the sphere surface emits also in the direction perpendicular to the sphere's radius, and those light beams don't all focus on the other foci.
But, for a big enough ellipsoid and small enough spheres, what you say is less and less relevant; each point on the surface is an omnidirectional (hemispherical) radiator but the effective sum in any one direction will be the same as from a central point; I think it is the equivalent to the gravitational potential of a spherical shell compared with a central point mass. My Watts per square cm flux idea still holds, I think.
I don't think that has resolved the paradox yet.
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it's not the same since the sphere surface emits also in the direction perpendicular to the sphere's radius, and those light beams don't all focus on the other foci.
But, for a big enough ellipsoid and small enough spheres, what you say is less and less relevant; each point on the surface is an omnidirectional (hemispherical) radiator but the effective sum in any one direction will be the same as from a central point; I think it is the equivalent to the gravitational potential of a spherical shell compared with a central point mass. My Watts per square cm flux idea still holds, I think.
I don't think that has resolved the paradox yet.
I'm not sure it solve the paradox too, but nontheless I'm not even sure that what you say is correct. It doesn't seem to me that the ellipsoid's dimensions are relevant; a bigger ellipsoid doesn't do anything else than to reflect light farther from where are the spheres; what counts should be only the relative dimensions of the two spheres.
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It must be ok to postulate an optical system which is near enough perfect to provide conditions where this paradox could seem to exist(?). My idea was to have a big enough reflecting system to allow the two spheres to be considered as isotropic radiators.
This is such an obvious problem that someone must have discussed and solved it already. We have to hope that some other forum member can put us right.