I was reading up on distortion of light due to air pressure differences and causing "puddle" mirages and fata morgana. Also the "shimmer" effect of rising hot air. Basically all the web sites said the reason for all this was the fact that the refractive index of air changes with air density. I haven't quite figured out exactly why a greater refractive index at the ground level would cause a mirror image, but I do get the basic idea of the concept and will work on a more thorough understaning of the machanics at another time. I do have a question, however. Maybe no one has figured this out yet and that is why I can't find the answer on any of these pages, but I thought I would ask here. Why does the refractive index of air increase with decreasing density? Is there a defined principle at work here?
Are you sure you are not confusing the effect of air <i>density</i> with the effect of air <i>temperature</i>? Even though the mass density is not generally consistent with the optical density, if you consider two uniform distributions of air at equal temperature, I think that the denser of the two will have a higher index of refraction. (I am also searching the net right now for the answer.)
Re: Re: Light slows in hot air? I am not confusing density and pressure. I mentioned hot air because, generally, hot air is less dense (therefore has a higher refractive index) which is what causes the "puddle in the road" mirage. Explain, please.
Ha? Shouldn't light travel faster via (at least) tranculent/transparent material with less density? And refraction should have little to do with speed of light.... it's just because the light is refracted all the time that it reached your eyes later.....
Am I wrong in assuming that Total Internal Reflection also plays a role in this "puddle in the road" mirage? As in the earth's atmosphere acting as a prism with the layer of hot air near the ground being the reflective surface of the "prism"? I am curious if anyone has ever placed a hot object in a vacuum to see if it had any of the "shimmer" refraction patterns above or around it at all. I have been searching the net for this, but I just keep coming up with pages talking about what refractive index is.
Okay good shot..... I just remembered Einstein's postulates; that speed travels at constant speed in VACUUM. Now think: Modern atomic models is described as hollow, meaning there is a lot of vacuum even in solid material, which means, there is nothing to stop light other than the 'massive particles' such as atomic nuclei. Now back on the refraction: As light always travels at constant speed, the only thing that makes it seem to travel slower via different materials is the particles residing in the material (sorry, I lost some terms)..... Since it is 'very close to impossible' for these atomic particles to line up straight, there is a high chance that light will hit these particles often-> reflection in atomic level, refraction in our level. higher temperature means extra chaos to this atomic lining up, meaning that the chance that light hit the particles increases greatly. I'm afraid that's a little unclear, so ask me about that which confuses you.... And about what you call Total internal refraction, I have no idea
No it is perfectly clear. The problem I see with that idea is that, as you said, "it is 'very close to impossible' for these atomic particles to line up straight", therefore there could not be uniform refraction. You also pointed out: "Modern atomic models is described as hollow, meaning there is a lot of vacuum even in solid material, which means, there is nothing to stop light other than the 'massive particles' such as atomic nuclei." Light photons, on the other hand are very small, right? So the majority of the photons will get through, while some will get reflected by particles within the atomic structure. However, that sub-atomic reflection wouldn't be refraction on the macro scale, it would be diffraction which is clearly evident when light passes through air.
I do understand (and somewhat share) your confusion, though. If light moving slower in the Earth's atmosphere is attributed to the matter in the atmosphere (which is what I was told) then an medium with MORE dense matter should slow it down more. There seems to be a missing piece of the puzzle. (or maybe just the two of us are missing that piece. Please Register or Log in to view the hidden image!)
OK. This is really beginning to bug me now. It seems that the more dense a medium is the greater the refractive index is: Material n at 589.3 nm (yellow sodium light) n (x-ray) vacuum 1 helium 1.000036 air 1.0002926 carbon dioxide 1.00045 water ice 1.31 liquid water (20°C) 1.333 glycerine 1.4729 rock salt 1.516 bromine 1.661 glass 1.5 to 1.9 diamond 2.419 I was told in the past that this has to do with atoms absorbing and emitting photons. The more dense the material is, the more closely the atoms are packed, therefore the more times it has to be absorbed and emitted by atoms, slowing it down. I am not sure if this explanation is correct, but it sounded reasonable enough at the time. Hot air, as we know, is less dense than cold air, right? It would stand to reason that light would travel faster in hot air, not slower but every site I have read (probably a dozen or two now) have stated that the puddle mirage is caused by the drastic change in air density within a few inches above the road surface. They say that light moves more slowly in hot air since it is less dense than cooler air, therefore having a greater refractive index. :bugeye: Please Register or Log in to view the hidden image! WTF? What is going on?
they said that the puddle mirage is caused by light slowed down by hot water? I can't say for sure.... but the only theory I know about it is that the closer air is to the earth during a hot day, the hotter (and thus less dense) it is. When light goes down in an angle from above, it is refracted to one extent that it gets almost parallel to the surface of earth. And supposedly, air layer which is too close to the earth gets far too thin, causing the downward refraction to fail to occur; instead, the light is then reflected upward and refracted so that it becomes less and less parallel to earth-> the puddle mirage is thus a reflection of the sky (or whatever up above). As for your comment, about the 'hard lining-up': Analogy: If you are to cycle between two rows of trees, can you go in a straight line if the rows of trees are not perfectly straight and parallel to your lane? Actually that was what I mean, but I don't know if either of us misunderstood the other. Oh, and I also want to contradict my theory by adding up yours to mine: Why is it that the refraction index of water is far less than diamond? In my theory, the more chaotic the particles in a matter is, the greater the refraction index-> perhaps I should have said diffraction index?
A wave travelling through a less dense medium will travel faster, if it is entering the change in density at an angle one side will travel faster than the other for a split moment whilst the other side is still in the denser air. So that the wave turns, this process continues as the wave travels downwards through lighter and lighter air and as one side of the wave is always in air of a lower density the wave will continue to turn and move back upwards untill it has left the area of fluxuation. Imagine a water ripple entering shallow water at an angle, it will turn towards the angle or if entering deeper water (hence easier movement) it will bend away from the angle.
I found something that looks pretty comprehensive. I am leaving work soon, so I don't have time to read it all right now. You can look at it in the meantime: http://www.glenbrook.k12.il.us/gbssci/phys/Class/refrn/u14l1a.html
Re: Re: Re: Light slows in hot air? Check again, I was contrasting density and <i>temperature</i>. Yes, I would agree with this in the given context. I found a site with this concept mentioned, but I don't think anymore that it's very important. Basically, optical density is directly related to the time it takes light to travel through the material (in other words, it is directly related to the index of refraction). Mass density you probably already know. The site mentioned that these two densities are not necesarily directly related/proportional/or something, but, I don't think that this is relevant to the concern you are having. My explanation of mirages: On a hot day, the ground can be very hot, hotter than the air temp measured by most thermometers (i.e. it could be 100 deg outside, while the concrete or sand is hot enough to burn the bottom of your feet in rather quickly, around 150 deg or so, just pulling a number out of my ass). I don't know if you have experienced this personally, but I will go ahead and vouch for it. In this situation, the air immediately above the ground can be considerably hotter than the air further above the ground. From what you have posted previously, I don't think that you'll disagree with the claim that this results in a decrease in air density closer to the ground. This in turn results in a decrease in the index of refraction closer to the ground, which in turn leads to the light traveling faster closer to the ground, which in turn leads to the light curving upwards from the ground if it is incident at sufficiently shallow angles, which in turn leads to you observing light from the sky hitting your eyes from the direction of the ground, which is what water does, but by means of reflection, a different mechanism than refraction (though maybe not fundamentally).
I was just noticing the high end refractives in one raven's list-- glass 1.5 to 1.9 is classified as a liquid and diamond 2.419 is a crystal lattice structure, isn't it? Might have something to do with it... Andre: You could see it with thermography though couldn't you? But you are concerned with light, not heat readings...
This is not my experience. It is definitely clear, and so, in a way, it is always just about invisible, but I have always been able to see the ice/glass in the water. Glass is almost invisible in the air (if it is clean, there have been jokes centered around this fact), and the difference in the indices of refraction is even more pronounced in this case.
It is generally accepted that a mirage is <i>not a reflection</i>, but that it <i>is a consequence of REFRACTION</i>. There is no point in the light's path at which it encounters a stark boundary between regions of different temperature. If you wanted to, you could talk about the phenomenon on the atomic level, but there is no need; it makes perfect sense in the macroscopic model of light (along with the idea that the index of refraction decreases with temp).
I'll look for a reference in my spare time (for air, I'll let you find one for water). I will say right up front that the index will be quite insensitive to temp (maybe on the order of ppm, I can't remember off the top of my head). I probably wont get this to you today.
