Forum menu
I never said that the speed of light didn't vary, I said that the speed of the protons won't.
They're photons mate.
Transmission of EM radiation in a solid is dependend on absorbtion. If there are no atoms to absorb the incident frequencies then the waves will carry on. Absorbtion and re-radiation as you describe it is a special case and would result in a beam of light being totally scattered all over the place, rather than the direct transmission we see with beams of light etc.
**** me, there's some smart arses on here! One bit of trivia, it's often cited that the Americans spent a fortune developing a ball-point pen to work in space, while the Russians just used a pencil. In fact the pen was developed privately and offered to NASA for free, the company working on the fairly safe principal that publicity would sell millions of pens and cover all costs and make a profit. They did.
Yeah, get a room bofs...... 😉
Tachyons can travel faster than the speed of light.
Anybody read that theory that the sun creates enough solar wind to deflect the solar storms from the rest of the galaxy to create our stable solar system.
The solar winds weaken due to distance from the sun at the edge of our solar system and there is huge solar storms preventing us from traveling across the galaxy for a while.
When light enters materials, its energy is absorbed. In the case of transparent materials, this energy is quickly re-radiated. However, this absorption and re-radiation introduces a delay. As light propagates through dielectric (for example)material it undergoes continuous absorption and re-radiation. Therefore when the speed of light in a medium is said to be less than c, this should be read as the speed of energy propagation at the macroscopic level. At an atomic level, electromagnetic waves (light) always travel at c in the empty space between atoms.
Whether you like it or not...it does, so there. 🙂
The Russians DID NOT use a pencil. The bits of graphite that break off the end would cause havoc in a zero gravity space capsule.
dave_aber - Member
First man to (allegedly) fly around the moon, whilst Neil Armstrong and Buzz Aldrin (allegedly) landed on it was Michael Collins. Not the Irish one though.
Ignoring the previous missions that had already been there. Apollo 8 for a start.
In the case of transparent materials, this energy is quickly re-radiated
disagree - it passes through. If it were re-radiated it would lose all collimation and lenses would not work. But the refractive index [b]is[/b] the ratio of the speed of light in the dielectric to that in free space.
Space 2008 - have it
[img] 
[/img]
(I know I'm lowering the tone of this thread!) 😆
And I repeat for those of you who simply don't,won't or can't understand the process:-
"Whether you like it or not...it does, so there"
I'm a particle physicist and JW is right.
(I know I'm lowering the tone of this thread!)
Yep, but a couple of things in that photo are more spectacular than anything I've seen in space 🙂
The fuel pump on the space shuttle at full power on take off could empty an
olympic swimming pool in one minute
disagree - it passes through. If it were re-radiated it would lose all collimation and lenses would not work. But the refractive index is the ratio of the speed of light in the dielectric to that in free space
Thankyou SFB. If jahwomble were right, absorbtion and re-radiation would result in scattering and beams of light would not stay straight, and lenses would indeed not work.
It's not that we don't understand what you are trying to say (I know about quantum levels, photon emission, re-radiation and all that, how neon lights work, streetlights work etc etc) it's just that you are wrong 🙂
Where do you get your information from btw?
As this is only my second post I'm going to try to to agitate anybody anymore than absolutely necessary.
Now I'm talking here at an atomic level. As light travels through transparent matter.... it "bumps into" the atoms (technically, the photons do in fact keep getting absorbed and re-emitted) and so the light can appear to travel slower in a suitable medium,without the photons slowing down. The correct arrangement of atoms will create an appropriate structure for refraction.
We measure this phenomenon with a number called the index of refraction, usually represented by the variable 'N'. N is defined to be the speed of light in a vacuum divided by the slower speed in matter, and it depends both on the type of matter in question (different atoms/substances absorb and emit light in different ways) and the wavelength of light in question (different wavelengths of light get absorbed and emitted at different speeds, even in the same substance).
This apparent slowing of a light wave is responsible for the way light bends as it enters mediums like glass and water. A cleverly shaped piece of glass (or other material) can take advantage of this property and bend groups of light rays to make an image appear larger or smaller. Such pieces of material are commonly called lenses, and are used practically everywhere.
Prisms work because of the fact that different wavelengths of light have different speeds in the material, and so get bent different amounts. The different colors composing "white" light get bent differently as they go through a prism, and the resulting separated colors form the rainbows you generally see coming out of prisms.
I don't supposed that has helped really has it?
335.geek - sorry, you're wrong.
In a transparent material, photons with the frequency range of visible light continue straight through. This is because the electrons are incapable of absorbing the lower energy photons of visible light. This is also why glass is opaque to UV light.
Refractive index is actually defined as a function of the material's relative permittivity and permeability, which relate to its resistance to forming electric and magnetic fields. The fact that it can be calculated from c/c(material) is merely a consquence of how the speed of light is defined.
Dave (Physicist).
This Vibek Saugestad video features a spaceman at one point
And I repeat for those of you who simply don't,won't or can't understand the process:-"Whether you like it or not...it does, so there"
if it were so how could diffraction effects - which depend on phase - occur ? Are you saying the re-radiation is still in phase in non-lasing conditions? That doesn't sound like absorbtion to me - resonance perhaps - but than can only occur when there is an energy level corresponding to the photon. And bear in mind white light is made up of multiple frequencies, each of which would require a co-resonant exitation mode in the substrate!
which relate to its resistance to forming electric and magnetic fields. The fact that it can be calculated from c/c(material) is merely a consquence of how the speed of light is defined
what are you actually saying here ?
I feel embarrassed posting the actual questions now.
1) Who was the first man in space?
2) Who was the first man to orbit the earth?
3) Apollo 1 and Soyuz 1 had what in common?
4) On what date did man first set foot on the moon?
5) What was the rocket for the Apollo missions?
6) Who was the first Britisher in space?
7) What was 'Buzz' Aldrin's real name?
8 What is the date of the next shuttle launch?
Nothing about photons I'm afraid.
I can't believe there were no questions about Vibek Saugestad's video. Your quiz was a bit rubbish I think
"Whether you like it or not...it does, so there"
also, Science doesn't work like that. The entire body of knowledge is theory, based on conjecture and measurement, and all available for refutation in the light of new evidence. There is no "it just IS".
I don't supposed that has helped really has it?/No, cos you're wrong and you just said what jahwomble said 🙂 Why would an absorbed and re-emitted photon be travelling in the same direction as the original one? Plus, if this were the case the speed of light in a medium would be directly related to its density would it not? And I do not believe this to be the case. Glass is pretty dense. Plus, why would some things be transparent and some not? Things with colour (ie most things) absorb certain wavelengths of light, right? So why are they not re-emitted? That would make everything non-reflective transparent to some degree.
SFB is spot on. For a change 🙂
which relate to its resistance to forming electric and magnetic fields. The fact that it can be calculated from c/c(material) is merely a consquence of how the speed of light is definedwhat are you actually saying here ?
He's saying this:
c = 1/??0?0
Epsilon and mu in that formula are the permittivity and permeability which measure how electric and magnetic fields behave in certan media. Consequently, since EM waves are the constant changing of electric and magnetic fields, the speed of propogation of those waves is related to them.
Curious as to where this idea comes from since two people have now come out with it.
Was the answer to 3 1?
So are they waves or particles, I'm confused? Some of you are banging on about particles, some about waves. Someone is talking through their hat.
Not sure if this helps the debate, but I found it while googling what jahwomble was trying to explain to me:
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.
-- from the [url= http://www.physicsforums.com/showthread.php?t=104715 ]Physics Forums FAQ (www.physicsforums.com)[/url]
Reading this reminds me why I spent Thursday afternoons skivving physics round at a mate's house where he fast forwarded horror vids to the "good bits"
c = 1/??0?0Curious as to where this idea comes from since two people have now come out with it.
Which idea is that?
Answer to 3 is that in both instances the crew were killed, on launch and re-entry respectively. Got given the Haynes manual for the Apollo missions for Christmas, should have known that. Just astonishing the application of all the physics, like the fact that the skin of the lunar module was essentially bacofoil.
Man in Space is a pub in Stoke-on-Trent.
Classis Simpson's script.
"Who was the first man to land on the moon,Homer?"
Homer "Apollo Creed"
Gets me every time. 😆
which relate to its resistance to forming electric and magnetic fields. The fact that it can be calculated from c/c(material) is merely a consquence of how the speed of light is definedwhat are you actually saying here ?
He's saying this:
c = 1/??0?0
what I meant was, he appeared to be saying that the speed of light in the medium was somehow irrelevant, whereas in fact it's necessarily locked to the other characteristics
Simon - that's exactly what I was trying to say but it was late and I was tired!
So are they waves or particles, I'm confused?
Either. It depends on how you look at it. All particles can be treated as waves if you want to. Particles and waves are something that we know from the big world in which we live; in the very small world things are very very weird and seem impossible to our 'common sense' which really is big world common sense.
Flaperon - I was wondering where the absorbtion-reradiation theory of light propogation came from.
Particles and waves are something that we know from the big world in which we live; in the very small world things are very very weird and seem impossible to our 'common sense' which really is big world common sense.
Ok, not a very good explanation, but I still don't get why some things are transparent and others are not? What's so special about glass? And then there's X-rays, they seem to go through almost anything, do they slow down or not?
What's so special about glass?
or air or water ? A substance can only absorb radiation if it has a mode of excitement of the right energy for that of the photon - which is related to its frequency (ie colour etc) to quite fine tolerances. For visible light these modes are mainly different electron 'orbits' within atoms and molecules - for infra red the interatomic bonds have the right resonant energies. So, in glass, air and other transparent media, there are no electron transitions of energies corresponding to visible light quanto, so they are not absorbed. (Bear in mind I'm being slightly vague with my terms for simplicity)
Barnes is right, but more simply put:
If the thing you are shining light into can 'resonate' at the frequency of the light, it soaks up the energy and nothing comes out. If it can't, then the light just continues along its way. If it soaks up some frequencies and not others, then you only get some colours coming through and you end up with like a red filter or whatever.
Xrays go through lots of things because they are very high frequency and not much stuff can resonate at those kinds of frequences; likewise radio waves go through a lot of things too becuase they are very low frequences.
Gee, thanks guys! So with X-rays and radio waves are there photons like there is in visible light? And if there is do the photons slow down when they are passing though a medium or not? Is the process the same you were talking about with the light in glass.
Radio waves, X rays, microwaves, ultra violet, cosmic rays, gamma rays, red light, blue light etc are all the same, just different frequences. The red-green-blue bit we can see is just a very narrow section of the whole electromagnetic spectrum. Radio waves are very very very much redder than red light, and x rays are very very much bluer than blue light 🙂 The process of being transmitted through things is the same as with light and glass, yes.
Photons do slow down.. although when we analyse light travelling through things we talk about it in wave form. Waves for travelling, particles for hitting things.
That's much better, I am getting there, slowly 🙂 ! So when the light, radio, (whatever) wave hits something that is transparent what happens? Does it somehow change from a wave to a photon, this is the bit I don't really understand? How exactly does is slow down without changing?
It doesn't change from a wave to a particle, it IS a wave and a particle.
It doesn't change from a wave to a particle, it IS a wave and a particle.
I'd be tempted to say it's neither and they are just our rubbish attempts to explain something that seems odd to us.
r979 - the 'wave' is really the distribution of probability that a particle (photon) might turn out to be in a particular place when it interacts with something. The 'particle' aspect is just our explanation of the fact that the energy travels in discrete packets of a particular amount of energy (you can't get less and if you have more it is always a multiple).
How exactly does is slow down without changing?
what do you change into when you slow down ?
It doesn't change from a wave to a particle, it IS a wave and a particle.
think of it as a packet of waves
Photons never change from particles to waves, nor does anything else. They are both at the same time. What you see depends on what experiment you are doing to try to find out. There are simple experiments that conclusively prove that light is particles, and equally simple ones that prove it is waves.
When talking about light travelling places, it's handy to treat it like a wave, and when talking about it hitting things it's convenient to talk about particles. That's all.
Let's say you own a road bike and a mtb. Saturday morning you are wearing lycra, no helmet, zipping about in a big bunch of riders not saying anything except making snippy comments about minor transgressions of group riding etiquette. Sunday on the other hand, you are out on the hills with a different group of mates wearing baggies, a helmet, grinning widely after each descent and spending ages sitting about on the hillside talking about singlespeeds and suspension. You are both a roadie and an mtber, and the way you behave and what you do depends on the people you are with and what they are doing... So it's kind of like that 🙂 When your roadie club looks at you they see a roadie, when your mtb mates look they see an mtber.
Which tyres for quantum physics?