Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: labview1958 on 18/11/2008 00:37:39
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The refractive index of copper is 1.10 as shown in
http://www.ps.missouri.edu/rickspage/refract/refraction.html
How? Copper is opaque. How can light pass through? How do they measure?
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Light can pass through very thin layers of metal.
The aluminized mylar film which is used in solar eclipse spectacles is an example.
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Can Infrared light pass through thick copper? If so, is it refracted inside thick copper?
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Can Infrared light pass through thick copper?
No, it gets reflected
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Can UV light pass through thick copper?
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No you have to go up in frequency until the wavelength is significantly shorter than the spaces between the nucleii which means pretty hard Xrays before copper and most other conductore become significantly transparent Then you do not have a conventional refractive index only a diffraction pattern off the reasonably regularly spaced nucleii
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Can infrared or UV pass through a thin layer of copper. Will the em radiation be refracted?
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Can infrared or UV pass through a thin layer of copper. Will the em radiation be refracted?
1. yes
2. yes.
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Could labview pass through a thick layer of copper?
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If UV or infrared can pass through a thin layer of copper. Does the uv/infrared beam get refracted or diffracted.
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If UV or infrared can pass through a thin layer of copper. Does the uv/infrared beam get refracted or diffracted.
Refracted.
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Always diffracted too, unless the aperture is infinitely wide and the atoms are infinitely small.
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When light passes through glass, why is it not diffracted but refracted?
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It will always be diffracted - the degree of diffraction will depend upon the geometry - aperture and molecular dimensions. In a 'block' of glass the aperture is enormous so the 'ray' is relatively unaffected in many cases. If you are using a glass telescope lens, diffraction is very relevant; it limits its resolution. Atoms are smaller than light wavelengths so they diffract the energy they intercept in all directions (scattering). The resultant wave pattern produces an undeviated ray - but diffraction must still have taken place.
The refraction will depend upon the electronic energy levels etc etc in the substance, which affects the phases and amplitudes of photons when they are absorbed and re radiated by the atoms. This affects the speed of the wave.
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If a light ray is incident at 90 degrees to the glass surface, it does not get refracted. Does it mean that light does not slow down in glass for this particular case.
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If a light ray is incident at 90 degrees to the glass surface, it does not get refracted. Does it mean that light does not slow down in glass for this particular case.
No. It means that light don't enter through it.
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If a light ray is incident at 90 degrees to the glass surface, it does not get refracted. Does it mean that light does not slow down in glass for this particular case.
No. It means that light don't enter through it.
(Not 90degrees to the normal)
The light is still slowed down but it can't be bent 'towards the normal' - it's already there.
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Well, I could throw a spanner in here and say that light always travels at c and that what we are in effect seeing is an absorption/re-emission delay.
If the energy of the photon corresponds exactly to the energy needed to 'bump' an electron to a higher shell then we have absorption. If it is, instead, close but not exact then we get a 'virtual' bump followed by a re-emission of the photon a small time later, as the electron returns to its normal 'shell'.
The closer the energy match, the more the time for the re-emission. The amount of time taken for the re-emission is a measure of the refractive index.
(This is a semi-classical, semi-quantum explanation. For a full quantum explanation then you need a bigger brain than mine :-) )
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I'd go with most of that except for your use of "shell". In a solid there aren' t discrete levels but bands (Pauli applies) and any frequency of wave can interact. The density will affect the delay because of the number of interactions per cm. In a gas the only 'simple' electron interactions can be at spectral lines. I think there must be some other effect to account for speed change - vibrating the whole atom, perhaps ?( neutral charge tho so that sounds suspect) If it were frequency dependent there would be strong dispersion. I'll have to read round.
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From another forum:
<<Do Photons Move Slower in a Solid Medium?
Contributed by ZapperZ. Edited and corrected by Gokul43201 and inha
This question appears often because it has been shown that in a normal, dispersive solid such as glass, the speed of light is slower than it is in vacuum. This FAQ will strictly deal with that scenario only and will not address light transport in anomolous medium, atomic vapor, metals, etc., and will only consider light within the visible range.
The process of describing light transport via the quantum mechanical description isn't trivial. The use of photons to explain such process involves the understanding of not just the properties of photons, but also the quantum mechanical properties of the material itself (something one learns in Solid State Physics). So this explanation will attempt to only provide a very general and rough idea of the process.
A common explanation that has been provided is that a photon moving through the material still moves at the speed of c, but when it encounters the atom of the material, it is absorbed by the atom via an atomic transition. After a very slight delay, a photon is then re-emitted. This explanation is incorrect and inconsistent with empirical observations. If this is what actually occurs, then the absorption spectrum will be discrete because atoms have only discrete energy states. Yet, in glass for example, we see almost the whole visible spectrum being transmitted with no discrete disruption in the measured speed. In fact, the index of refraction (which reflects the speed of light through that medium) varies continuously, rather than abruptly, with the frequency of light.
Secondly, if that assertion is true, then the index of refraction would ONLY depend on the type of atom in the material, and nothing else, since the atom is responsible for the absorption of the photon. Again, if this is true, then we see a problem when we apply this to carbon, let's say. The index of refraction of graphite and diamond are different from each other. Yet, both are made up of carbon atoms. In fact, if we look at graphite alone, the index of refraction is different along different crystal directions. Obviously, materials with identical atoms can have different index of refraction. So it points to the evidence that it may have nothing to do with an "atomic transition".
When atoms and molecules form a solid, they start to lose most of their individual identity and form a "collective behavior" with other atoms. It is as the result of this collective behavior that one obtains a metal, insulator, semiconductor, etc. Almost all of the properties of solids that we are familiar with are the results of the collective properties of the solid as a whole, not the properties of the individual atoms. The same applies to how a photon moves through a solid.
A solid has a network of ions and electrons fixed in a "lattice". Think of this as a network of balls connected to each other by springs. Because of this, they have what is known as "collective vibrational modes", often called phonons. These are quanta of lattice vibrations, similar to photons being the quanta of EM radiation. It is these vibrational modes that can absorb a photon. So when a photon encounters a solid, and it can interact with an available phonon mode (i.e. something similar to a resonance condition), this photon can be absorbed by the solid and then converted to heat (it is the energy of these vibrations or phonons that we commonly refer to as heat). The solid is then opaque to this particular photon (i.e. at that frequency). Now, unlike the atomic orbitals, the phonon spectrum can be broad and continuous over a large frequency range. That is why all materials have a "bandwidth" of transmission or absorption. The width here depends on how wide the phonon spectrum is.
On the other hand, if a photon has an energy beyond the phonon spectrum, then while it can still cause a disturbance of the lattice ions, the solid cannot sustain this vibration, because the phonon mode isn't available. This is similar to trying to oscillate something at a different frequency than the resonance frequency. So the lattice does not absorb this photon and it is re-emitted but with a very slight delay. This, naively, is the origin of the apparent slowdown of the light speed in the material. The emitted photon may encounter other lattice ions as it makes its way through the material and this accumulate the delay.
Moral of the story: the properties of a solid that we are familiar with have more to do with the "collective" behavior of a large number of atoms interacting with each other. In most cases, these do not reflect the properties of the individual, isolated atoms.>>
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That's nicely put.
'The Hydrogen Atom' model which we all get taught is all too often used out of context and the Energy Levels in solids are much more complicated.
The quotation doesn't deal with the behaviour of light passing through a gas, though. Gases have very low refractive indices but the effect is still very broad band (except at absorption peaks). I wonder how that can be explained, in similar sorts of terms; it can't be due to mechanical vibrations - can it?
Having said that, the effect of em waves through ionised media can be treated as forced oscillations of the free electrons in a classical way.
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Yes, that beats my (oversimplified) attempt at an explanation hands down [:)]
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When light passes through a glass prism it breaks down to it's rainbow colours. Why when light passes through a glass block, it does not break down to it's rainbow colours?
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When light passes through a glass prism it breaks down to it's rainbow colours. Why when light passes through a glass block, it does not break down to it's rainbow colours?
Because light passing through a prism is forced to refract in the same direction when it enters and when it escapes; passing through a glass block no: it is refracted in a direction when it enters and refracted in the opposite direction when it escapes, so the total refraction is zero.
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You can get chromatic aberration, though - the blue 'ray' is shifted laterally a bit more than the red.
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You can get chromatic aberration, though - the blue 'ray' is shifted laterally a bit more than the red.
That's true. Given the angle of incidence, the shift is proportional to the glass' thickness.
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Does the refractive index of glass varies with the frequency of light?
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Does the refractive index of glass varies with the frequency of light?
Of course, otherwise you wouldn't have dispersion (= separation) of colours with glass.
There are types of glass or other materials where the dispersion is lower, but there is always. A material with very low dispersion is CaF2 (calcium fluoride) in the crystal form of fluorite; for this reason it's used in good, but expensive, optical instruments as high quality camera's lenses or telescopes' objectives (to have almost no chromatic aberration). Such materials are called apochromatic.
http://www.daviddarling.info/encyclopedia/A/apochromatic.html
http://en.wikipedia.org/wiki/Apochromat
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Does it mean red light travels faster than blue light in glass? Is there any material where blue light travels faster than red light?
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Does it mean red light travels faster than blue light in glass?
Of course.
Is there any material where blue light travels faster than red light?
In the right-side of the anomalous refraction curve of any substance (that is, on the right of the absorption band): wavelenght on the orizontal axis, index of refraction on the vertical axis.
See "anomalous refraction" or "anomalous dispersion".
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If I shine a light on the moon, would the red light reach the moon a nanosecond earlier than the blue light.
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If I shine a light on the moon, would the red light reach the moon a nanosecond earlier than the blue light.
Yes, if you shine the red light a nanosecond earlier than the blue... [:)]
They arrive simultaneously. Why do you think they could arrive at different times?
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This assumes that the Earth Moon path is a pure vacuum which of course it is not.
The red and blue light has to pass Thru both the atmosphere and the ionosphere and a differential delay could well arise.
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This assumes that the Earth Moon path is a pure vacuum which of course it is not.
The red and blue light has to pass Thru both the atmosphere and the ionosphere and a differential delay could well arise.
Correct. I assumed labview1958 intended to talk about a travel in the void, considering Earth's gravitational field.
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As the moon is moving towards/away from earth, a light from earth would show a nanosecond blue/red shift. This is due to Doppler effect. Agree?
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As the moon is moving towards/away from earth, a light from earth would show a nanosecond blue/red shift. This is due to Doppler effect. Agree?
If you are on the Moon observing a light beam from Earth, you will see it blue shifted if the Moon approaches Earth and red shifted if the Moon recedes from Earth.
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Is it possible to design a "prism" in such a way that blue light is transmitted out while red light is internally reflected.
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As the moon is moving towards/away from earth, a light from earth would show a nanosecond blue/red shift. This is due to Doppler effect. Agree?
If you are on the Moon observing a light beam from Earth, you will see it blue shifted if the Moon approaches Earth and red shifted if the Moon recedes from Earth.
But the effect is tiny.
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As the moon is moving towards/away from earth, a light from earth would show a nanosecond blue/red shift. This is due to Doppler effect. Agree?
If you are on the Moon observing a light beam from Earth, you will see it blue shifted if the Moon approaches Earth and red shifted if the Moon recedes from Earth.
But the effect is tiny.
Ah, but it depends on what he intended with "if the Moon approaches Earth"; if he intended "during the actual phase of Moon's approachement to Earth" or if he could mean "in a Moon launched at relativistic speed towards the Earth" (because I haven't understood it) [:)].
Anyway, the effect is perfectly detectable with modern interferometry (even with no relativistic Moon [;)]).
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Is it possible to design a "prism" in such a way that blue light is transmitted out while red light is internally reflected.
Yes, it should be possible, chosing the right frequencies of red and blue, the right material and a suitable range of internal incidence angles.
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I think you would need a material with a negative coefficient of dispersion, though.
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I think you would need a material with a negative coefficient of dispersion, though.
Yes, that was part of "the right material". Note however that you have to choose the material if you have already fixed the frequencies, but you don't need to do it if you don't fix them: any material have a negative slope part of the dispersion curve, exactly from the other side of the positive one, with respect to the absorbtion frequency:
http://phys.strath.ac.uk/12-370/sld081.htm
So, you can take glass, identify a frequency of absorption (with coloured glass this frequency is in the visible) and then choose the two frequencies on the part of the curve where the dispersion coefficient is negative.
Simpler than what we could think, isnt'it?
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"On the other hand, if a photon has an energy beyond the phonon spectrum, then while it can still cause a disturbance of the lattice ions, the solid cannot sustain this vibration, because the phonon mode isn't available. This is similar to trying to oscillate something at a different frequency than the resonance frequency.
So the lattice does not absorb this photon and it is re-emitted but with a very slight delay.
This, naively, is the origin of the apparent slowdown of the light speed in the material. The emitted photon may encounter other lattice ions as it makes its way through the material and this accumulate the delay."
How exactly is this photon 'delayed'?
It will move at 'c' in 'any space' and there is a lot of space available in atoms no matter what matter.
In a BEC you slow photons by pushing it in the direction it came from,
Is it this that means, or is like getting through some sort of 'density' created by the electron clouds.
That is, if we look at photons particle wise.
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I think you would need a material with a negative coefficient of dispersion, though.
Yes, that was part of "the right material". Note however that you have to choose the material if you have already fixed the frequencies, but you don't need to do it if you don't fix them: any material have a negative slope part of the dispersion curve, exactly from the other side of the positive one, with respect to the absorbtion frequency:
http://phys.strath.ac.uk/12-370/sld081.htm
So, you can take glass, identify a frequency of absorption (with coloured glass this frequency is in the visible) and then choose the two frequencies on the part of the curve where the dispersion coefficient is negative.
Simpler than what we could think, isnt'it?
I have an idea that the graph in the link refers to the classical group delay characteristic of a minimum phase bandpass filter (digging back in my memory). Does it still apply to a piece of coloured glass?
I think that 'coloured glass' - if you just mean pigmented glass - wouldn't do the same job. The bulk of the medium would still be the basic glass so would the total phase shift be due to a combination of the two materials? It's not just a simple resonator.
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As the moon is moving towards/away from earth, a light from earth would show a nanosecond blue/red shift. This is due to Doppler effect. Agree?
If you are on the Moon observing a light beam from Earth, you will see it blue shifted if the Moon approaches Earth and red shifted if the Moon recedes from Earth.
But the effect is tiny.
Ah, but it depends on what he intended with "if the Moon approaches Earth"; if he intended "during the actual phase of Moon's approachement to Earth" or if he could mean "in a Moon launched at relativistic speed towards the Earth" (because I haven't understood it) [:)].
Anyway, the effect is perfectly detectable with modern interferometry (even with no relativistic Moon [;)]).
Which effect is measurable?
The movement of the moon- (which is fairly small, but fairly easilly measurable) or the doppler shift that this tiny movement produces (which I'm pretty sure would get swamped by the variable effect of the earth's atmosphere?
Incidentally, if you are prepared to take a rather wide interpretation of the word "prism" then dichroic filters and transmit and reflect just about any colours you want.
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I am sure that an interference filter could achieve any phase characteristic you wanted, in principle, BC. But, to achieve the same sort of delay characteristic corresponding to the TIME dispersion in a prism, you would need a lot of layers, I think.
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I have an idea that the graph in the link refers to the classical group delay characteristic of a minimum phase bandpass filter (digging back in my memory). Does it still apply to a piece of coloured glass?
I think that 'coloured glass' - if you just mean pigmented glass - wouldn't do the same job. The bulk of the medium would still be the basic glass so would the total phase shift be due to a combination of the two materials? It's not just a simple resonator.
From the book "Fundamentals of Optics" - Jenkins/White - fourth edition - paragraph 23.4 "Anomalous dispersion":
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The existence of a large discontinuity in the dispersion curve as it crosses an absorption band gives rise to anomalous dispersion. The dispersion is anomalous because in his neighborhood the longer wavelenghts have a higher value of n and are more refracted than the shorter ones. The phenomenon was discovered with certain substances, such as the red fuchsin and iodine vapor, whose absorption bands fall in the visible region. A prism formed of such a substance will deviate the red rays more than the violet, giving a spectrum which is very different from that formed by a substance having normal dispersion. When it was later discovered that transparent substances like glass and quartz possess regions of selective absorption in the infrared and ultraviolet, and therefore show anomalous dispersion in these regions, the term "anomalous" was seen to be inappropriate. No substance exist which does not have selective absorption at same wavelenghts, and hence the phenomen, far from being anomalous, is perfectly general. The so-called normal dispersion is found only when we observe those wavelenghts which lie between two absorption bands, and fairly far removed from them.
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When light passes in air it is in "deep" space compared to when it passes through glass, it is in "shallow" space. Thus the light bends giving rise to the refractive index. Thus deep/shallow space depends on the medium.