Metals reflect light becuase the atomic structure of metals allows for free electrons-which set up an opposing electric field at the surface-hence reflecting light.
As an insulator, glass has few free electrons-so why do you get reflections from it?
Glass is not merely an insulator, it’s a dielectric . Its electrons, while not free to flow, can shift in response to an electric field – in a word, it’s polarizable (the same is true of water). The difference between reflection from a dielectric and reflection from a conductor is demonstrated by the fact that light reflected from glass is polarized, and light reflected from metal is (mostly) not.
So, how do you get a reflection off polished metal?
Of course, everything is a dielectric. The only reason that glass is more specular than most substances is that it’s a lot smoother.
I don’t know the answer, but I would point out that most insulators, indeed, most solids, reflect light, i.e., they are opaque.
Something can be opaque without being reflective, if it absorbs light.
I was under the impression that no true black body actually exists. All actually existing opaque substances reflect some light (although they all also absorb some light too, as, indeed, do transparent substances). Are you suggesting otherwise?
The same way you get down off an elephant.
(For which you might use a henfur or a Grecian Urn.)
I was mostly just suggesting that opacity does not imply reflectivity, and there are certainly substances which are a good approximation to black. But if you really want to nitpick, a black hole is a perfect blackbody, with complete absorbtion and no reflectivity.
I’m certainly not foolish enough to contradict you in this field, so please take this as a question:
How does Hawking radiation fit into that? Isn’t it related to the mass of a black hole? Since energy and mass are interrelated (they are, aren’t they?), wouldn’t a black hole that takes in large amounts of radiation over time increase in mass? If it increases mass, wouldn’t it emit more Hawking radiation? Or is that semantically cheating?
Incandescent re-emission of radiation is not the same thing as reflection. And a black hole with a greater mass actually produces less Hawking radiation.
Go find “Applied Electromagnetism” 2nd Edition by Liang Chi Shen and Jin Au Kong and refer to Chapter 4, in particular pg73-80 (Reflection and Transmission at a Dielectric Interface).
Snarky yes, but it’s been one of those days.
You are the one who is nitpicking. All actually existing opaque substances (and, come to that, transparent ones too, apart from colorless gases, maybe) reflect some light and absorb some (the transparent things transmit some too). Some, like matte black things, may absorb a good deal more than they reflect, others, such as white things, reflect much more. The point is, however, that reflectivity does not depend on electrical conductivity, as the OP implied. Some insulators are highly reflective, and all of them are somewhat reflective.
The black hole is beside the point. If you say a black hole counts a black body, I am prepared to take your word for it. However, by my understanding, it is not a substance in the sense that glass, or metal, or most opaque things are, and I rather doubt if the concepts of insulator and conductor even apply to it. (And even if you want to insist that it is a substance, and is an insulator, that still does not invalidate my point. It would still remain the case that very nearly all insulators reflect light, and some reflect quite a lot of it.)
Yes, but your original statement,
says that reflecting light and being opaque are synonymous, which is far from the truth. Yes, everything (other than black holes) reflects light to at least some degree, and everything is opaque to at least some degree, but there’s no particular relationship between the two.
When a light wave encounters a transparent surface (no natual frequency in the visible range), the wave goes through and comes out the other side. Glass has few free electrons, but has some. If a certain matter contains free electrons that are capable of vibrating at any frequency, it will scatter light in all directions.
When a light wave hits a surface with free electrons, such as a metal, it reflects. If the surface is rough, the reflection will be many different directions, but a smooth surface (such as glass) reflects according to the simple law of angle of incidence equals angle of reflection.
OT:
You know, nitpicks can be frustrating as all hell. In most instances, they really, really suck. Threads can so easily get derailed by two or three posters in a round-robin *I’m right *… **no, I’m right **… *no, I’m right *… no, I’m right… never realizing that from a third party point of view, they’re all right and instead of simply recognizing that they’re talking past each other or calmly acknowledging either the mis-wording or exception someone is highlighting, or simply validating the underwhelming correctness of the others, they’re all acting like idiots in their myopic focus to prove not only that they’re right, but the other is wrong (er, there’re lots of other outcomes and situations, this is just the first that came to mind. I don’t mean to characterize anything here but my limited stream of consciousness).
(pause for breath)
Not to mention that the posters themselves must find it awfully frustrating to get caught up in a nitpickket’scharge.
Sheesh! :rolleyes:
But
Once in a while, with a MUCH higher frequency on the Dope than anywhere else, even though the picked nits are still nit-worthy or tend towards the arcane (though blindingly obvious to those students of the arcane), the resulting back-and-forth honing of nuance is rich in information and fascinatingly colorful. It doesn’t mean all technical nitfights are worth it, but some are, and some deserve to be acknowledged. Thanks.
It doesn’t have free electrons by any standard definition of “free electron”. As has been pointed out, specular reflection is a pretty inherent property of materials. I suspect that even gases can reflect at a phase boundary between two gases. (or gas and a vacuum)
I’ve been staying out of this because I thought it had been answered early on. And I don’t need to compulsively nitpick, as Rhythm was complaining about, but I have to point out that this really isn’t true.
Most metals are opaque to light, and will specularly reflect if smooth. But very thin “leaf” of various metals will, in fact, let visible light through. And alkali metals are actually transparent in the ultraviolet.
Unless the gases have very sharp boundaries, I doubt it. more likely is that , with their ease of diffusion, one gas will gradually tail off into the other, and you won’t see a reflection. Pretty much the same as when you put two miscible liquids together, or if one liquid has a concentration gradient of salt or sugar in it. The index varies gradually, and you get interesting bending of the light, but no reflection.
You will, though, get reflection at the well-defined boundary between two immiscible liquids, or between a liquid and a solid or gas.