Metals don’t have diffuse reflection, only specular reflection. I’m wondering about the light reflection and refraction properties of materials that are in-between being metals and non-metals. I understand that you can’t give me an exhaustive answer; I’m looking for principles, data points and ballpark figures.
What about metalloids (e.g.: silicon, boron)?
Or cermets (e.g.: titanium carbonitride)?
Or metal matrix composites (e.g.: tungsten carbide)?
What if you take a metalloid and make a metalloid matrix composite with it (e.g.: boron nitride, silicon carbide)?
Wow, this is way past what I studied in ore microscopy. In that subject, we studied mainly thick sections of metal sulfides and oxides (and maybe some free gold.) I tried googling the optical properties of the elements and compounds you mentioned but I’m not getting much. All I can give are basic guides if you ever get around to studying them on a suitable microscope. The most basic question is whether or not a specimen is transparent or opaque. Most of what you mentioned are opaque or translucent, and therefore require a reflecting microscope.
The first thing you should observe from a polished section of an opaque or translucent substance on a reflecting microscope is color. The megascopic color of your specimen is often repeated in reflected light but not always. Color is affected by the second basic observation, which is reflectance. Reflectance is the ratio between incident light (striking the specimen) and reflected light. The amount of reflected light could vary with the degree of polish possible for a specimen. This is influenced by physical properties such as luster, hardness, texture, and uniformity. Certain combinations of all of the above will be diagnostic for a given specimen.
Reflectance and color in non-metals can also vary with orientation as you rotate the stage. This is because the polarized light from the light source will bounce off differently from the specimen, depending on its orientation relative to the crystal structure, or mineralogical/petrographic texture. In this respect, it’s somewhat easy to identify a pure metal as its color and reflectance will be uniform regardless of orientation. And therefore changes in reflectance (bireflectance) and color (pleochroism) are additional diagnostic features of a given specimen.
The possible observations given above use only plane polarized light. If you add a second polarizer (analyzer) with direction at right angles to the first, you will enter a second phase of microscopy wherein you study substances under “crossed polars.” I can’t go into that at length but suffice to say they are meant to further provide diagnostic features for a given specimen and certain peculiarities within just one type.
“The most general mechanism by which a surface gives diffuse reflection does not involve exactly the surface: most of the light is contributed by scattering centers beneath the surface”
“Few materials don’t follow it: among them are metals, which do not allow light to enter”
“These materials can reflect diffusely, however, if their surface is microscopically rough, like in a frost glass (Figure 2), or, of course, if their homogeneous structure deteriorates, as in the eye lens.”
Note that in that type of diffuse reflection, the light would not be colored by the material. It’s less diffuse reflection than specular reflections at a smaller scale than the incident ray; Like a highly coherent jet of water that splits up into several high coherent jets of water instead of splashing around.
Also, aluminum can be considered a metalloid rather than a metal; There’s some debate about this.
I think the reason metals don’t have (ordinary) diffuse reflection is because the constant sharing of electrons makes light either bounce right off the very surface without even penetrating a few microns or gets completely absorbed. If I’m wrong about this, I’d love to be corrected.
Tungsten carbide, in the form you normally encounter it in (such as a drill bit or end mill) is indistinguishable from any other gray metal. It is dominated by the specular component. However, when broken you get an extremely diffuse surface, like a rough powder. I’m not sure if the difference is entirely due to the surface roughness or if there’s something else going on.
You’re probably not looking at a single crystal of tungsten carbide but a formation of tungsten carbide and cobalt in a matrix of pure tungsten. The resulting pattern will be peculiar in an electron microscope.
The reason I’m asking is because I was listening to videos/reading guides on computer graphics and it was mentioned that materials either have a Metal_Value of 1 or 0 with no materials in-between like 0.1, 0.5 or 0.9 and that if a material did have a Metal_Value in-between 0 and 1, it would be some kind of alien material.
So I’m wondering how the materials I mentioned above look and interact with light since it seems like they would behave like something in-between a metal and a non-metal without being alien.
How does silicon interact with light?
What makes the metals and non-metals on the line between the metal group and the non-metal group in the periodic table behave so strangely?
I think the reference to computer graphics sounds more like a simplifying assumption for people making games or something.
I believe the big distinguishing feature of metals in optics is that their electrons form a free “electron gas” that moves easily around in the solid, and can do things like create an image field to counter the electric field details outside of them – in other words, when electromagnetic radiation approaches a metal, canceling fields appear inside it to satisfy the condition that there’s no potential gradients in the metal (or at least very little in this context). This causes a reflection. But it isn’t true that there’s no opportunity for color; copper is orange, osmium is blue, gold is yellow, and many other metals have a tinge (often slightly yellowish). And I’m talking about pure metallic elements, not their oxides or something.
Metals also have some transparency, just not much. Thin films of gold are used, for instance, to provide some filtering. Gold in this sense is very strongly green. Being transparent, metals have an index of refraction, too, and it’s typically very high. Interference filters use this property of metals.
A more complete description of the optical behavior of a solid is expressed by its complex index of refraction, which has a real part describing refraction and an imaginary part describing absorption. The two numbers are not free to vary independently. There’s a relation between them as a function of wavelength - I think it’s called the Kemper-Koenig relation or something similar.
About silicon and other metalloids, they have free electron gas only under some conditions, and they look somewhat like a metal and somewhat not. It’s not that special. Better to think of the traditional metals as a simple extreme situation where typically only the field canceling behavior applies, and the traditional nonmetals as a simple extra situation where typically only ripples in the constituent electron positions convey what in free space we’d call light.
1.) You certainly can have diffuse reflection fro metals. If the surface isn’t perfectly smooth, you can get breaking up of the light and multiple reflections, causing the same kind of light pattern you get from a diffuse white surface. MEM cites texts claiming that some degree of penetration into the material is necessary, but it ain’t necessarily so. In addition:
2.) Light does penetrate into metals. It just doesn’t penetrate very far. This depth is called the “skin depth” The skin depth varies with wavelength, and, of course, with the metal. For copper, the skin depth is about 0.0000006 cm for 10 micron light, 0.00006 cm for microwaves, and 0.006 cm for long radio waves (wavelength 1 km)
3.) silicon and germanium, semiconductors, optically behave like other dielectric materials (including glass) in partially reflecting like and partially transmitting it, refracting the incoming light. The thing is, the refractive index is much higher than things like glass, and in the visible has an imaginary part, which makes it absorptive. In the infrared, they act much like glass, but with a really high index.
4.) Metals have real and imaginary parts to their refractive indices as well. but the imaginary part is pretty large. Nevertheless, the same effects – differebnt reflections of different polarizations, Brewster angle polarization – can be seen with metals, too.
5.) The optics of other materials, particularly with non-smooth surfaces, gets complex. To tell the truth, I still don’;t understand why it is that Teflon is white. It’s near-relative, polychlorotrifluoroethylene, can be made so that it’s perfectly transparent. Or, processed another way, it can be as translucent as Teflon. It’s not clear to me why Teflon doesn’t behave the same way.
In thin pieces, Teflon is practically transparent – take a piece of Tefloon plumbing tape and immerse it in pure ethanol and it becomes transparent. The ethanol matches the refractive index of the Teflon, filling in the surface irregularities that make it white. TYhe effect goes away when it dries. But it’s not all surface scattering – that thin piece of tape has internal scattering, as well. Although you can see anything it’s lying on clearly through it, if you hold it further away, it’s like tryinmg to see through waxed paper.
Teflon is the perfect case of what MEM wrote about scattering happening inside the material. Because of this multiple scattering, Teflon is almost the ideal Lambertian diffuse reflector. Companies like LabSphere sell Teflon-coated surfaces and Teflon-coated Integrating Spheres as Diffuse Reflectance standards.
Sounds like to a first order approximation those blended materials will have metal-like reflective behavior as a function of their electrical conductivity.
Confounding factors:
Degree of surface roughness at the scale relevant to our incident light’s wavelength.
Grain size.
It’s also worth emphasizing that the magic terms “diffuse reflection” and “specular reflection” are about atomic-scale electronic properties, not about how a macroscopic sample looks to human eyes. The micro-differences have macro-consequences, but the situation is not as neatly categorical at the macro scale as all that. Except in computer animation software.
Teflon, or better to say polytetrafluoroethylene (because Teflon is a trade name for several different polymers), is birefringent. And it does not exist in any forms I know of that have total crystalline order over a scale of more than maybe a few micrometers. I think that keeps it more or less white in all circumstances, even if it has no void spaces.
Hi
As far as I know, Teflon is only polytetrafluoroethylene, even if some people call other things “Teflon”
Is it birefringent because there is a different index along the chain vs. perpendicular to it? If so, then why is properly processed polychlorotrifluoroethylene transparent? Inquiring minds want to know.
I wondered about the possible differences between what the computer graphics software was doing and the actual science, hence why I asked.
If I’m getting the info in this thread right, even metals like flat polished iron or copper plate should not have a Metal_Value of 1 because the software treats them as black diffuse-wise if I understand the software(s) correctly. The software(s) in questions are the physically based rendering (PBR) shader(s) used in Substance Designer/Painter and Unreal Engine 4. It provides results like this in real-time which only used to be possible in pre-rendered mode:
So they should have a Metal_Value of 0.9 to 0.99 rather than 1 if “1” treats the diffuse as black, right?
Non-metals should also be getting a value above 0, even if slightly, because they have some canceling field too, right?
I did find it strange when “no diffuse color” was mentioned because I immediately thought of copper which obviously has a color.
Do metals tend to slightly color specular reflections? If I shine a pure white light at a plate of copper, will the specular highlight that bounces off of it be slightly orange-ish white?
So, what’s the most extreme metal when it comes to this conversation? What about the most extreme non-metal?
A rough plate of pure iron could produce light as diffuse as, say, wood, skin or marble?
Was that last figure extrapolated or did someone send a 1km-long wavelength into copper?
So if you looked through a silicon plate with IR goggles, the plate would have the effect of badly-adjusted eyeglasses?
So, as a rule of thumb, if conductivity is 100%, it should get treated as having a Metal_Value (i.e.: no diffuse color) of 1. If conductivity is 50%, a Value of 0.5 etc?
I’m sure the degree of conductivity gets more complex than a simple percentage but this is what my non-EE degree holding, non-ore microscopy-studying self can approximate with my current knowledge on this topic. I started the thread with the expectation that I’d learn more.
I can’t say; beyond my expertise. I’d be amazed if it was a linear percentage through. Which is why I weaseled by saying it was “a function of conductivity” without specifying *which *function.
OP, before you go into dept about optical properties, can’t you begin with chemical/atomic and megascopic properties? It’s strange to be discussing birefringence when nearly all anisotropic and some plastics have it.
I don’t know about experiments sending long radio waves into metal. It’s certainly possible. But I strongly suspect the figure I give (which is from Born and Wolf) is the result of plugging the wavelength into the formula for skin depth.
Absolutely.
I’ve done this, in fact. You can see through a polished piece of silicon with Night Vision goggles, or a Find-R-Scope.
If you have one of the long wavelength Find-R-Scopes you can see through Germanium, too, but its cutoff wavelength (around 1.8 microns) is too long for standard night vision (Silicon cuts off light below about 1.2 microns).
Weird fact – things happen down the other end of the visible spectrum, too. The Alkali Metals – Lithium, Sodium, Potassium, Cesium – become transparent in the ultraviolet. Of course, they’re highly reactive, too. But if you had a thin layer sandwiched between quartz plates you’d have an effective low-wavelength-pass filter.
Puts a whole new light on Transparent Aluminum, doesn’t it? (except that aluminum isn’t really transparent in any wavelength regime close to the visible, AFAIK.)
