Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: chris on 13/01/2010 23:23:01
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At the level of a photon stream, what is happening on the surface of a mirror to enable it to reflect light the way that it does?
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You mean, why can a mirror bring us back an reflection but a blanket won't?
That's a really good question.
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It is because there is a conducting surface in which the em wave induces a changing current. I'm afraid I have forgotten the details, but vaguely remember that the "reflected" wave has a 180 degree phase change. The wave cannot penetrate into the conductor because the electrons flow to prevent any potential difference being built. The energy is reflected back. This is not a very satisfactory answer. I am not sure reflection can be explained by considering light as a stream of particles. It has to be the wave aspect that works here, although I suppose you could consider conservation of energy and momentum
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If we look at a wave then it passes through the glass, meets the 'silvery surface' and as that is highly reflective (Up to 99.99~ % in the best mirrors)it then sends the wave back through the glass to your eye. The light that comes back have first meet you and all other things around you and then the mirror and then your eyes. The reason why a smooth even piece of cloth won't reflect the light is that it absorbs it instead, transforming it into other wavelengths (heat mostly).
And its the smoothness of the material that decides the angle it will reflect back, the smother the material the more precisely will it follow the same angle at which it hit the object. And metal is very smooth. The reason for the 'quicksilver/silver/aluminum coat' is that metals shares their electrons with all metal atoms, but glass doesn't, which means that metal easier frees an photon than glass does. That is, the 'free electrons' in metals do the work, releasing the photons.
As for why it can carry a image of you?
That's a hard one, but all reflections do so, the screen you're looking at is also a reflection of light, even though it doesn't reflect you. All materials reflect some light to your eyes but why mirrors reflect your image instead of just the material must have to do with that it interacts very little with the material in the mirror, getting released immediately, therefore keeping the information it had when reflected from you, as a guess that is :)
But I would also like a better answer to that one?
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Maybe a silly question but is a reflection you see in the mirror a true reflection of how you look?? do the waves of light travelling towards the mirror interact with the ones returning after interacting with the mirror and therefore distort the image that reaches your eyes??
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Ah, I found this one too. It seems as it should be shared :)
------Quote---
There have been many cases of people installing 2-way mirrors in female changing rooms. It is very difficult to positively identify the surface by just looking at it. So, how do we determine with any amount of certainty what type of mirror we are looking at?
Just conduct this simple test: Place the tip of your fingernail against the reflective surface and if there is a GAP between your fingernail and the image of the nail, then it is a GENUINE mirror.
However, if your fingernail DIRECTLY TOUCHES the image of your nail, then BEWARE... FOR IT IS A 2-WAY MIRROR!
So remember, every time you see a mirror, do the "fingernail test." It doesn't cost you anything. It is simple to do, and it might save you from getting visually raped!
REMEMBER: "NO SPACE, LEAVE THE PLACE!"
---End of quote---------
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So what's the difference between a mirror and a piece of white paper?
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They don't interact as photons at least?
And very weakly as waves if I remember right?
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So in essence, what we see in the mirror could be slightly distorted fromour actual appearance?
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The texture/structure will spread the reflection of the paper, shooting of in all kind of directions, also it has to do with what type of atoms there is and how they bind their electrons.
And depending on how it absorbs it will release different 'wavelengths', creating different colors as we get 'hit' by those waves/photons.
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Take transparent glass f ex.
If you crush it it will no longer be transparent, but will look white to you.
That is a direct result of it getting reflected, instead of just passing through as light did when it was whole.
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How do you mean doppler1?
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If we go back to the material reflecting (mirror), or rather, if seen as photons interacting with atoms, releasing new ones to your eye.
"For the visible light range, you will notice that the most efficient reflectors are metals. The free electrons in the metals are the ones predominantly involved in reflecting these photons. And to be able to account for that, one needs to understand the electronic band structure of metals, what solid state physicists call the dispersion curve (E versus k graph) of metals. And then, you have to understand the reduced or extended zone scheme to show why only certain k vectors or momentum are allowed in such a transition when a photon is absorbed in the process before it can be re-emitted as a reflected photon with the same in-plane wave vector."
And that make my head hurt..
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So what's the difference between a mirror and a piece of what paper?
Err, I thought I answered that.
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Back to the original question about the photon-nature of mirrors:
The idea of a "photon stream" striking a mirror is a subtle one. See my post here (http://www.thenakedscientists.com/forum/index.php?topic=27814.msg295295#msg295295) and ligharrow's post that follows. The problem is that light isn't simply a stream of photons. You can write reflection as an interaction between an initial photon and a mirror that gives as output a reflected photon and the mirror. I don't know the details, however. I do know that it's not as simple as just having the photon bounce off the mirror, since photons don't zip about like tiny bullets.
The far easier-to-use model is what graham.d suggested: that you model the light as a classical wave.
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I've always liked light :)
I used to have an extremely good book about light and color and its nature, I think she (the author) had an Italian (?) name. There wouldn't be any chance that someone here recognize my description. It was a incredibly easy read, as well as very good, considering the subject.
As for waves, I'm still wondering, like I think Chris was, where the difference comes in between those two cases. The one where we have a mirror 'remembering' the information contained in the waves/photons giving you back your image versus the one where light also are reflected from a smooth surface but without presenting you that image?.
And thinking of it, how about those electrons??
Take a window, dark outside, light inside. It reflects too, doesn't it? So there have to be 'free' electrons mobilized there too?
But the author, if you know tell me :)
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I always understood that the best mirrors as used in gas discharge lasers used a multiplicity of non conducting dielectric layers so electrons cannot be the answer.
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Ah, I found this one too. It seems as it should be shared :)
------Quote---
There have been many cases of people installing 2-way mirrors in female changing rooms. It is very difficult to positively identify the surface by just looking at it. So, how do we determine with any amount of certainty what type of mirror we are looking at?
Just conduct this simple test: Place the tip of your fingernail against the reflective surface and if there is a GAP between your fingernail and the image of the nail, then it is a GENUINE mirror.
However, if your fingernail DIRECTLY TOUCHES the image of your nail, then BEWARE... FOR IT IS A 2-WAY MIRROR!
So remember, every time you see a mirror, do the "fingernail test." It doesn't cost you anything. It is simple to do, and it might save you from getting visually raped!
REMEMBER: "NO SPACE, LEAVE THE PLACE!"
---End of quote---------
Thanks for that.
Next time I'm installing a two way mirror I will set it up so that it passes this test to lull people into a false sense of security. (It will still work as a 2 way mirror.)
They work by being a poor mirror. The important thing is that the "secret" side of them is poorly lit.
The dielectric mirrors on lasers work due to refraction of light and that is also due to the electrons in the material.
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About the OP question. I agree with JP and graham. The photon has not an individuality because it's an excitation of the EM field, so it's not like a ball the bounce back from the mirror: there is an EM field propagating towards the mirror and the field has excitations called photons, then it interacts with the mirror, a new field is generated as consequence, it results that this new field is propagating backwards, this field has excitations which are called photons.
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So what's the difference between a mirror and a piece of what paper?
The slightly oversimplified answer is that the important difference is that the mirror is smooth and the paper is not. For specular reflection to occur, light needs to be scattered the same way regardless of where it strikes, which requires that the surface be smooth at least on a scale comparable to the wavelength of light.
What makes this answer oversimplified, is that while a smooth surface is necessary, it isn't sufficient. If the surface is smooth on this scale, but there is subsurface irregularity on the scale of the wavelength, reflection will be primarily diffuse rather than specular. This is reflected (no pun intended) in the fact that if you polish a white stone to a glossy finish, it's still white.
The conducting nature of the metal in a mirror does explain the high efficiency of reflection, but any material that is sufficiently smooth and orderly will display specular reflection in whatever it does reflect. That's why a smooth piece of glass can display a good reflection when it is brightly lit on one side and poorly lit on the other.
there is an EM field propagating towards the mirror and the field has excitations called photons, then it interacts with the mirror, a new field is generated as consequence, it results that this new field is propagating backwards, this field has excitations which are called photons.
OK, while this is a useful view for some purposes, you need to be careful with it. Because you can do an experiment where you turn down the intensity of the incoming light to the point where one photon at a time is incident on the mirror. If you measure where the reflected photons come off, you will see them, one photon at a time, at an angle of reflection equal to the angle of incidence.
Now like I said, the description is useful. If we think about a classical electromagnetic wave, and we consider how it scatters off of a mirror's surface, after we crunch through all the details we'll find that the wave interferes destructively everywhere except coming out an angle of reflection equal to the angle of incidence. But when we think about that classical wave, we are thinking about part of the wave that reflects off one part of the mirror interfering (constructively or destructively) with part of the wave that reflects off another part of the mirror.
Now if we apply this picture naively to the quantum wave, we can be tempted into thinking of it as one photon that reflects from one part of the mirror interfering with another photon that reflects from another part of the mirror. And if we allow ourselves to think of it that way, it is actually not intuitive that specular reflection will occur for a sufficiently low intensity source.
We have to view the situation as the individual photon interfering with itself. The photon acts as if it takes all possible paths toward and away from the mirror, and these paths interfere with each other with the only path that doesn't cancel out being the one we expect from the classical view.
It can be tempting to consider this an obvious consequence of the quantum mechanical view of light; after all we associate the wavelength of the classical wave with the wavelength of the photon. And it would have been a bigger shakeup of quantum physics if low intensity (i.e. single photon) reflection didn't mimic classical reflection. However, I think it is worth maintaining our sense of surprise here. While the behavior is what we expect from classical theory, the best explanation we can come up with is weird from our classical viewpoint.
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I like Feynman's 'many paths', even though I have some problem combining it with the idea of a one way 'arrow of time'. On the other hand, this is the quantum view of 'reality', and there the arrow seems to become discussable. "Feynman’s concept was that, in a sense, a particle follows all possible paths, and it just so happens that the lengths of nearly straight paths are not very sensitive to slight variations of the path, so they all have nearly identical lengths, meaning they have nearly the same phase, so their amplitudes add up. On the other hand, the lengths of the more convoluted paths are more sensitive to slight variations in the paths, so they have differing phases and tend to cancel out. The result is that the most probable path (by far) from A to B is the straight path."
Maybe one could look at as some weird sort of 'constant'?
Or would that be totally out of the question?
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there is an EM field propagating towards the mirror and the field has excitations called photons, then it interacts with the mirror, a new field is generated as consequence, it results that this new field is propagating backwards, this field has excitations which are called photons.
OK, while this is a useful view for some purposes, you need to be careful with it. Because you can do an experiment where you turn down the intensity of the incoming light to the point where one photon at a time is incident on the mirror. If you measure where the reflected photons come off, you will see them, one photon at a time, at an angle of reflection equal to the angle of incidence.
In such a case you can't simultaneously measure both angles for a single photon.
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OK, while this is a useful view for some purposes, you need to be careful with it. Because you can do an experiment where you turn down the intensity of the incoming light to the point where one photon at a time is incident on the mirror. If you measure where the reflected photons come off, you will see them, one photon at a time, at an angle of reflection equal to the angle of incidence.
In such a case you can't simultaneously measure both angles for a single photon.
You know something? You can't simultaneously measure both angles for a classical wave either. Any part of the wave that you intercept to measure its direction doesn't make it to the mirror. You are taking it on faith that the parts that you don't intercept are in fact behaving the same as the parts that you do, and that pulling your detector out of the way makes no difference. I'm not criticizing this; it's a damn good assumption, but don't pretend it's not being made.
And anyway, what we really do when we perform this kind of experiment is that we position our source and aim it. We base our angle of incidence upon measurements of the physical setup of the experiment, not on a direct measurement of which way the light is travelling before it reaches the mirror.
What you can do with photons, and what is a perfectly legitimate experimental set up, is that you can start with a collimated photon source. You can measure the output profile of that source. Then you can aim it at a mirror. You will know the angle of incidence to within an experimental error based on your first measurement, which lets you make a prediction of the angle of reflection with an acceptable margin of error. You can then
Has anyone done that experiment? I'll confess that I don't know. I do know that interference and diffraction experiments have been done with a photon-at-a-time source. I know that classical optics essentially treats specular reflection as a special case of diffraction. And I know that modern astronomers rather rely on their telescopes (which use mirrors) being able to make images of stars and deep-sky objects even when the light from those sources is coming in one photon at a time. Do you have serious doubts that specular reflection works the same when you turn down the intensity of your source to the point that you get one photon at a time?
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No. What I say is that the particle/photon paradigma doesn't explain anything here.
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Since the silver backing is responsible for the reflection what does the glass add to the mirror? I'm assuming that the glass clarifies and/or removes the the metallic quality from the reflection. If so how?
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Since the silver backing is responsible for the reflection what does the glass add to the mirror? I'm assuming that the glass clarifies and/or removes the the metallic quality from the reflection. If so how?
The glass does several things:
1. It allows the deposition of a very thin film of metal, saving metal.
2. It provides a very smooth and plane reflecting surface.
3. It protect the metal from wear, scratches and chemical corrosion and provides a very rigid media for the reflecting surface.
4. It allows to use the metal surface and not its unperfectly transparent oxide layer (in the case of Aluminum for example).
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I think the best explanation of how a mirror works was made by Richard Feynman. I believe the explanation is covered in his video lectures available here
http://vega.org.uk/video/subseries/8
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Sorry for butting in but---I just want to know---why does a mirror reverse things left and right but not up and down and how does it know the difference?
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Sorry for butting in but---I just want to know---why does a mirror reverse things left and right but not up and down and how does it know the difference?
Keep in mind that "right" and "left" are concepts defined relative to an individual, while "up" and "down" are concepts defined relative to the surface of the Earth. This is, I think, where most of the confusion comes from in this question. You can just as easily ask "When I'm talking face-to-face with someone, why do we agree on which directions are up and down but disagree on which directions are left and right?"
To see what's really going on in a mirror, you need to think about directions that are on the same footing. So for the sake of argument, let's suppose you have a mirror on the north wall of your house. When you look at yourself in the mirror, your up and down are your image's up and down, your east and west (directions on the same relative footing as up and down) are your image's east and west, but your north is the mirror's south and vice versa.
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Fred,
The reason it works that way is because the mirror does not actually reverse anything. The image in a mirror is a true reflection of reality. Relative to you, up is up and right is right.
When you look at a photograph, the image in the photograph was taken from a completely different position in space, so what you see is relative to that position in space.
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The image in a mirror is a true reflection of reality.
WHAT??? No, no, no. This can't be true......(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fbestsmileys.com%2Fcrying%2F2.gif&hash=6f40ed49d250203da654520072aa4687)