first off i don't have a clue about physics. ok with that established, here's my question
how come radiowaves can go through walls and lightwaves cannot?
ty in advance for answering
first off i don't have a clue about physics. ok with that established, here's my question
how come radiowaves can go through walls and lightwaves cannot?
ty in advance for answering
First off i'am like you I not an expert so dont take this as concrete fact but I would guess its done to the frequencey / wavelength. Its the same for radiation some types can pass through pretty much anything except special shielding and some types can be stoppped by a sheet of paper.
(yes I know that light and radio waves are also types of radiation but i'am using laymans terms!)
Some light can pass through walls though for instance infra red can.Or has tv lied to me again? I remember the scene in robo cop where he sees the guy holding people hostage through a wall.
P.S Welcome to the board!
thank you for your answer and kind welcome. i know radiowaves have a a bigger wavelength than lightwaves but could someone please be more specific as to the exact workings. why does larger wavelength mean it can go through a wall?
I can not answer your question, but I can observe that since I have aluminum siding on my house, radio and TV waves have a difficult time coming through that metal. I have to run a radio antenna outside the house so I can get a better short-wave signal, and I have to do that with my TV antenna too.Originally Posted by Bernard2
I’ve heard that a satellite dish won’t work if you put it in an attic that has a metal roof on the house. Also, when it rains on my outside satellite dish, my TV picture breaks up and sometimes goes out completely.
I have a radio-transmitter outdoor thermometer, and I have to keep it near a window so the radio signals can come into my house. It has trouble transmitting through the aluminum siding.
Because the material in the walls doesn't readily absorb the radio waves. (This isn't meant to be smart-alecky, even if it sounds that way.)
All materials absorb radiation at frequencies determined by the electronic structure of their constituent molecules. We use glass as windows because it doesn't redily absorb photons in the visible range, but we choose the material for wall based on other criteria (e.g. looks, availability and structural properties. And we don't actually want light to go through!)
Metals tend to be really good at absorbing radiation in that range, as are (generally) any materials that have a large number of states for electron excitation.
Do atomic clocks have to be insulated and shielded to keep out unwanted EM waves? If so, if they were not shielded, would some EM waves alter the “tick rates” of the clocks?Originally Posted by swansont
OK, so radio waves (fotons) just go through the empty space of most bodies, sometimes bouncing against electrons, but not having enough energy to really move these electrons up to another level? In other words, they do not get absorbed because the energy cannot be USED in any way, something like that?
But I don't understand, because the fotons will at least move the electrons to which they bounce and loose their energy that way? While light fotons, which have more energy, will likely have a bigger chance to survive all the bouncings and have enough energy left to cross the entire body?
Well, obviously I don't understand this at all.
The fact is that I don't understand at all what a WAVE is. Yes, it's some sort of electronic or magnetic FIELD, in which a particle (foton) moves, but this foton moves in a straight line.
Now I know that we must regard light as sometimes being a wave and sometimes a particle, but what is JUST a wave? What is waving then?
BTW, I know this other guy, Bernard, we were chatting about this earlier.
Basically that. A photon is only useful if it has the exact energy needed to move the atom to an excited state (possibly the infinitieth excited state which involves ionisation). If no transition is requires the energy of incident photons, then it will pass through with little incident.Originally Posted by Sandor
That's the classical idea. But the quantum idea is all about packets of energy. If a transition between energy states requires 100eV, the classical idea is that if a wave containing 150eV comes along, it will do the excitation thang and carry on with 50eV left. But according to quantum theory, which is more accurate at this level, a 150eV photon will not have any effect because it doesn't have the exact energy level required.Originally Posted by Sandor
I believe the way to interpret it is that a large number of photons travelling together will approximate to a wave like thing.Originally Posted by Sandor
If I understand the situation correctly, there are two different theories about “what is waving”. I think the most common theory is that it is the electric and magnetic fields that are “waving”, and they zip through space at 186,000 mps as they “wave”. In other words, a light bulb and a radio transmitter generate these tiny little alternating electric and magnetic fields, and they "wave" or "wiggle" as they travel through space.Originally Posted by Sandor
The other theory is the minority opinion, and it says that what “waves” are the electric and magnetic fields that are already everywhere in space, and that all a light bulb and a radio transmitter does is start the local electric and magnetic fields to “waving” at the light bulb and the transmitter, and it is the “wave”, not the fields, that go zipping through space. This is a type of “ether” theory, so it is not very popular among physicists, although they will often talk about the “waves”, rather than the “fields”, traveling through space.
Originally Posted by Glom
The way I understand it is that a bunch of little waves traveling together in a “wave packet” group, constitutes a single “photon”. And so, several different groups of little “wave packets” constitute several “photons”, with one group out front and the others following that one. I don’t know if this is correct, but this is the way I imagine what the physicists are saying.
With this idea, a large number of photons traveling one after another, constitute a light “ray” or “beam”. While a lot of them traveling side by side, constitute a wide light “wavefront”. If a large number travel side by side, and more are following behind those, then we have a wider “beam” or “ray” of light.
It is the individual “wave trains” of several photons traveling in a straight line, one behind the other, that constitutes a very narrow “beam”. And when a light bulb generates a lot of photons in all directions, then a lot of individual “beams” diverge and go out in an expanding “spherical” manner, and this is what the “inverse square law” describes, that is, the gradual divergence of all these individual narrow photon “beams”.
In other words, the individual “beams” of single photon “wave trains” get further and further apart, the further they move away from a light bulb. Because of this phenomenon, a flashlight has a curved mirror behind the bulb, which changes the direction of some of these photon “beams” and focuses them to all go out together in a fairly straight line in the direction in front of the flashlight bulb.
A single photon:
A single photon “beam”:
Several photon beams coming out of a flashlight as a “light ray”:
Glom, that makes sense! It's the 'quantum amount' that will determine if there is no interaction at all, in an energy-kind of way.
Sam, I still don't understand, because an electric field is only measurable (or maybe even existing) when MATTER is involved, so a particle must be IN this field. or doesn't it (in light theory)? I mean, when they say that light can EITHER be regarded as wave OR as particle, how can it be ONLY a wave? Or is that not what they mean then?
Sam, that also was what I was thinking: I guess it cannot be that 1 photon is really 'something', or a particle, because they say that a radio photon has less energy (longer wave) than a light photon, but all photons have no mass (or not detectable) and all photons travel at the same speed (speed of light), so one photon cannot have more or less energy than another.
But the shorter the wave, the more photons will be fired in 1 time frame, so we can say: the number of photons in 1 time frame is the energy of the 'photon'.
What you are missing here Sandor is that photons have momentum which, because a photon has no rest mass, is the photons energy. A photon of visible light wave has more momentum than a photon of a radio wave.Originally Posted by Sandor
Tensor, but momentum is mass x velocity. Velocity is always the same and there is no real mass, or what? Mass is increasing when velocity increases, but velocity of all photons is the same and there is no mass? I don't understand.
Originally Posted by Sandor
You are using the Classical (Newtonian) formula. In Relativity, momentum for massless particles is defined as p = E/c Where p is the momentum, E is the total energy, and c is the speed of light.
Under relativity, the total energy of a system increases (not mass) as velocity increases, but the total energy also includes momentum as well as mass. In the case of photons, because they have no rest-mass, the total energy of a photon is equal to its momentum.Originally Posted by Sandor
OK, but can you explain why a light photon has more momentum than a radio photon? The electrical/magnetic wave 'around it' is shorter, so there are more waves in 1 space frame, but is this wave actually 'pulling it' forwards? Photons have no electrical charge I believe... (I'm just guessing around)Originally Posted by Tensor
I read that it is impossible for an object with any mass to travel at the speed of light, because when it comes closer to c, mass will increase, and it will increase so much that you will need all the energy in universe and more to get it at c. I understand what you're saying about ENERGY increasing and not mass, but the explanation above doesn't make sense then.Originally Posted by Tensor
An analogy for an electromagnetic wave is an audio wave-the peaks and the troughs represent amplitude, the number of frequencies represents pitch. The difference though is that lower frequency wave can past through walls much easier than higher freqs, it's just the opposite with electromagnetic waves.
Yes, they have to be shielded for a few reasons. EM radiation will cause an AC Stark shift in the clock states, so you either have to ensure they aren't there or measure them if they are. It's not easy, since room temperature blackbody radiation is a significant effect for the best clocks.Originally Posted by Sam5
Static magnetic field cause Zeeman shifts as well. Even though you use the transition that has zero magnetic moment (no linear shift) there is a second order effect. You put a known bias field in place to split the levels, since you don't want all of the states to mix - they are degenrate in zero field. The key is that it's a known bias, so you know the frequency shift. You shield out the earth's field and the associated fluctuations as best you can.
Photons (like all massless particles) have all their energy in momentum. As a result, higher energy photons (eg light) have more momentum than low energy photons (eg radio).Originally Posted by Sandor
Do you know about wave-particle duality. Waves or particles (a photon is the particle aspect of an EM wave) are different aspects of the same thing. An analogy would be vapor and dry ice are different aspect of carbon dioxideOriginally Posted by Sandor
Photons carry the EM force from one particle to another, but (and you are correct here) have no charge themselves.Originally Posted by Sandor
The mass increasing expanation you've read is a simplified explanation (as a matter of fact, mine is also simplified, though less so). The two explanations can be somewhat reconciled if you remember energy and mass are related by c (E=mc^2) I can, if you want, post how relativistic equations modify an objects acceleration thus preventing it from reaching c.Originally Posted by Sandor
Well, Sandor, have you ever heard of E = hf ? (where h is planck's constant & f = frequency)Originally Posted by Sandor
It's that simple; the greater the frequency, the higher the energy. 8)
momentum p = E/c = hf/c ...........the greater the frequency (shorter wavelenth) means greater the momentum.
Originally Posted by Sandor
I don’t know for sure, but this is the way I look at it:
If a photon is a small single “packet of wavelets”, this “packet” can have an “impact” on, let’s say, a very sensitive scale, and can give the impression that it has “weight”, even though it is only a “wave packet”.
Look at it this way. Get yourself a big bowl of water, then drop a penny in the water. You’ll see the water move in reaction to that “particle” or “group of particles” touching it.
Now, put one of your big hi-fi speakers up near the surface of the water, then turn up your hi-fi full blast, and look at the water. The surface is moving as if you are touching it. So, a “sound wave” is causing the water to physically move, and thus the sound waves are acting like "particles".
Ok, so you say that what is actually causing the water to move is “molecules of air” that are touching and vibrating the water. But, the last I heard about this subject, the “particles” of air don’t actually ever touch anything at all. They don’t even touch each other, ever. What causes a sound wave is not the particles of air bumping into each other, but the “fields” around the air molecules “compressing” each other. I’m not sure what these “fields” are called, but it is they that “compress” and “expand” to form a sound “compression/vacuum” wave in the air. So, a sound “wave” can have a “particle nature” too.
What we are dealing with on the atomic level, the last I heard, aren’t even “physical things”. They aren’t even “particles”, they are “wave forms” or, more properly, “field wave-forms”. So, my view of light is that each photon is a “packet” of tiny wavelets or “wave-forms” that, when they “hit” something, ie, when they rapidly come in contact with the tiny fields that surround other “wave-forms” that we usually call “matter”, the light photon tends to act as if it is a tiny “particle”. But, if I understand this quantum stuff correctly, even the “matter” the photon hits is not “matter” but tiny little “wave forms”.
I hope I haven’t confused anyone. I’m still trying to figure out some of this stuff myself.
I’ve got a 1932 book here, written by an early theoretical atomic physicist, who said, “We have thus been brought to the view that the whole of the material universe is built up of electricity and of nothing but electricity, the atoms of which are aggregated in an infinite number of different ways in the infinite number of different forms in which matter appears.”
So, if this is true, light is a special form of “electricity”, and so are you and I.
Originally Posted by Sandor
I think that is probably the correct view. The way I understand it, a photon is a long and skinny “thing” and so is a radio wave. Each have a “wave length”, and that determines how long they are. Even if all they are, are two vibrating or oscillating “electric and magnetic fields”, they seem to be long and skinny, with physicists saying the short ones have the most “energy”, because more of them “hit” per second. The physicists seem to ignore the “amplitude” in all this, but that’s their business I guess.
By the way, I finally figured out how radio and TV waves get down our antennas and into our TVs and radios. Actually, they just pass by our antennas, and the ones that “hit” our antennas are “absorbed” by the antennas. This process of “absorption” creates some “energy” inside the antennas, and causes the antennas to “resonate”, if the antennas are the physical length of the “wavelength” of the wave (or a certain percentage of the wavelength, such as a “quarter wave” antenna).
Ok, this “resonation” generates some kind of electron flow or “charge” flow or “current” flow in our antennas. Inside our radios and TVs, there is an electronic thing called a “detector” (or something like that) that detects the “current” flow from the antenna. Other stuff inside our radio and TV “amplifies” that weak current flow by duplicating its “variations” using the higher voltage and amperage coming from our batteries or the AC line that our radios and TVs are plugged in to.
So, the way I understand it, the radio and TV waves never make it past our antennas. It’s the “bump, bump, bump, bump” of the waves against our antennas that cause the antenna “resonation” and that resonation generates the electron or “current” flow inside the antennas.
If any radio expert thinks I’m wrong about any of this, then please point out my errors and tell us what actually happens.
Originally Posted by swansont
Great information, thanks!
Now, let me ask you this question:
How much will a temperature change inside the clock change the clock’s rate, as compared with how much the clock’s rate is affected by a change in elevation?
For example. If we note an atomic clock rate in the lowest earth valley, then we take the clock up to the top of Mt. Everest and note the rate change due to the elevation change, then how much of a temperature change would it take for the temperature to affect the “tick rate” of the clock in that same amount?
Hmm, this seems to be a space and time related thing. A 1-inch wave will have more “energy” than a 2 inch wave, because the 1 inch wave is carrying all of its energy past a point in space at twice the rate as a 2 inch wave. A hundred-meter wave will take much more time to carry all of its energy past that point in space.Originally Posted by Gsquare
So, with the Doppler effect caused by motion away from an observer, a 1 inch wave can be redshifted into an observed 2 inch wave, and, therefore, physicists will say that the original photon has “lost energy” in the process of being “redshifted”. Is that correct?
Whereas in sound and audio, the “amplitude” of the sound wave is more important and is considered to represent the “energy” of the sound wave, whether it is Doppler shifted or not??
Ok, so, when the earth is moving toward a distant star that is “fixed” relative to the sun, and the light of that star is “blueshifted” as a result of the earth’s motion around the sun, the incoming photons “gain energy”, so where does this “energy gain” occur, and where does the blueshift occur? At the earth, in the space between the earth and the star, or at the star?Originally Posted by Gsquare
OK, so it's really a matter of 'either' then. There's no photon moving in an EM field. It's either the photon or the wave.Originally Posted by Tensor
I would be glad to, Tensor.Originally Posted by Tensor
Russell & Gsquare, you give me the formulas or the 'knowledge as is', as being very 'simple', and I'm sure they are, but I want to understand HOW it works exactly.
Now it seems that 'momentum of a photon' is really the number of photons or waves per time frame, at least, this is what Sam and I think and it makes sense. Don't you think so Tensor?
It will raise other questions to me, but I still have some questions about 'waves' anyway. Russell and Sam were also mentioning sound waves and amplitude, but these 'waves' are only a graphical picture of what is really a longitudinal wave or movement. We can state the same here as for light: the number of molecules per time frame -or the wave length- determines the pitch, while the 'amplitude' of this graphical wave is really the maximum density of the compressed air compared to the minimum density of the air at a certain point, or the difference between those opposite situations.
This compression of air determines the power with which the molecules bounce in our ear. One would think that a higher compression (louder sound) will speed up the sound, but that's not true, at least as I understand it, because the speed is only depending on the 'matter' that is goes through.
Now, if we use this wave theory for light, we have indeed those 2 different factors:
1. number of photons per time frame
2. momentum or energy of 1 photon
The first one can determine the frequency (color) and the second one...well, what is that? The brightness or intensity of light? I guess the intensity is determined by the 'number of light sources'. Even if you use one lamp, the filament in it will glow at more points when the wattage is higher? So the more photons we see coming from 'one area', which is really a lot of 'points' close to each other, the more intensity we see, while a lot of photons coming from 'one point' will determine the color.
Now I still cannot find a meaning for the energy of 1 photon, but maybe I can for radio signals. Isn't it true that radio transmits at one frequency? So you have to tune at this frequency, which will really determine how many photons will arrive per time frame, you cannot change that. So how can it hold any data, like pitch and loudness? One would think that every single photon should hold different information. So you need an 'amplitude' there.
But what is an amplitude in a light wave? What is even a light wave, I must keep asking. It is only a graphical representation of numbers, but with sound, we can imagine molecules bouncing, getting compressed, speed up and slow down etc. What is the wave line of the electrical wave? Is it positive charge decreasing to zero, changing to negative charge, increasing until the maximun, decreasing again etc? How can it decrease and increase energy? They say the changing electrical charge causes the magnetic wave, simultaneously, or almost, because one thing causing another cannot happen at exactly the same time as the other thing, because it's CAUSING the other thing. The magnetical 'charge' again causes the electrical, but how can it go from positive to negative, or even passing zero and have no charge at all? How can this wave be interpreted?
Sam, I think you're right about the smallest particles not even being particles, but some sort of energy, of 'moving nothing', just like these light waves. No quark can be 'captured', and by the time we can 'see into' these quarks, we will maybe again find empty space with a much smaller 'part' in it, seemingly moving around etc.
The difference between energy and matter is just the space in which it moves: is it moving at relatively (compared to us or anything else) the same space (matter), or is it moving at greater distances (energy)?
Also I agree on the photon, which, if we have to 'materialize' it, should be a long skinny thing, because it has to 'fit in the whole wave'. One reason for this is that emitting the photons from the source will only send out one photon out if the other one completely left the source.
Interesting to read about the antennas. It tells us also why metal is 'shielding' the radio waves: because it absorps them.
About the Doppler effect: when the object is moving away, it sends out photons with the same frequency as it does when it is not moving away, that is, from it's own perspective. Because of relativity, we will see this frequency going down (time of a fast moving clock going slower). If we want to understand this in 'matter', the distance 'between' 2 photons sent out is longer, which in fact means that the photons are longer, or the waves. We then understand that there is not really any loss of energy (or gain of energy in blueshifting), but it is all a matter of perspective (relativity). When we move in the same direction as the emitting object, with the same speed, we will ourselves gain energy in relation to the photons coming our way, or the photons will gain energy, resulting in a 'higher' color.
There is no amplitude as far as I understand. The energy of 1 light ray is the color or wave length. Or maybe there is, but what is it?