Why do things have the color they have?

[quasar987]

I'm currently learning in my E-M class about the phonomenon of absorption, and I'm trying to figure out why we see what we see, but there are gaps in my understanding.

Regarding insulators: Depending on the composition of the material, there are various "absorption spikes" that will "kill" the waves of certain frequencies as they try to penetrate the material. But the material is pretty much transparent to to rest of the frequency spectrum. For exemple, since paper appears white, I conclude that there are no absorption spikes in the "visible range" of frequencies. And I can also speculate that the sky is blue because every visible frequency appart from blue is absorbed by the atmosphere.

But absorption cannot be the whole story, because for a metal, as a wave tries to penetrate it, every visible frequency is attenuated approximately equally and for all practical purpose, dies completely imediately after penetration. So the caracteristic color of metals must come from a phenomenon happening as the wave reflects from the surface!

I am aware that the index of refraction for a metal and a dielectric is frequency dependant, but I don't have time to see how it makes the reflection coefficient behave, so I thought it'd ask on PF for a qualitative description. Are there frequencies that are entirely transmited? If so, are they the same that are entirely absorbed (in the case of a dielectric), etc.

Thanks for any insight!

 

[Office_Shredder]

Actually, the sky is blue because blue light from the sky is scattered at a wider angle (yellow light is focused by the sky, which makes the sun look yellow)

 

[quasar987]

Scaterred huh? Do you mean "Compton scatered"? Or are you giving as an explanation that as the wave enters the atmosphere (which is a material of different index of refraction than vacuum), the wave is transmited in the atmosphere at an angle depending on the index of refraction, which is itself a function of frequency. So that is why blue is scatered at a different angle than the rest?

In either case, how does the blue light being scatered at a wider angle affects how we see the color of the sky? I mean, if there is no absorption, then all the colors should eventually reach our eyes regardless of what route they take.

 

[Hurkyl]

In either case, how does the blue light being scatered at a wider angle affects how we see the color of the sky? I mean, if there is no absorption, then all the colors should eventually reach our eyes regardless of what route they take.

No, they shouldn't. Unless you're looking directly at the sun, all of the light is going in the wrong direction, and should zip right past the Earth.

 

[OlderDan]

Scaterred huh? Do you mean "Compton scatered"? Or are you giving as an explanation that as the wave enters the atmosphere (which is a material of different index of refraction than vacuum), the wave is transmited in the atmosphere at an angle depending on the index of refraction, which is itself a function of frequency. So that is why blue is scatered at a different angle than the rest?

In either case, how does the blue light being scatered at a wider angle affects how we see the color of the sky? I mean, if there is no absorption, then all the colors should eventually reach our eyes regardless of what route they take.

Actually, it is Rayleigh scattering. It is not refraction. If there were no scattering by the atmosphere we would see the night sky all the time.

http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html

 

[quasar987]

Ok ok let's forget about the sky and get back to the metal case. What gives the metal its color? Why is copper brownish for instance?

 

[OlderDan]

Ok ok let's forget about the sky and get back to the metal case. What gives the metal its color? Why is copper brownish for instance?

Selective reflectivity and absorption. A perfect mirror reflects everything, with equal angles of incidence and reflection. Polished copper reflects some wavelengths more than others. You can still see an image, but the color is not true.

Unpolished surfaces reflect light selectively, but also diffuse it so the light leaves the surface in all directions. Perfectly white paper reflects everything diffusely. You don't see images; you just see white. Blue paper reflects primarily blue light and mostly absorbs other colors.

Color perception is a complicated business. Your eye has three different kinds of receptors with different frequency response curves. One of them is fairly well localized to the blue end of the spectrum. The other two have huge overlap with peaks in the green and yellow region of the spectrum, but both of them cover almost all of the visible spectrum. The one peaked at yellow is more sensitive to reds than the one peaked at green, but both are quite broad. Your brain process the intensity levels from these three kinds of receptors and tells you that you are seeing some color that may or may not be the same as some true monochromatic light. A hydrogen discharge tube produces only a few very distinct visible wavelengths. Your brain tells you the color you are seeing is something different from anyone of those individual components. Some of us are considered colorblind because we see different mixtures of wavelengths as the same color, while other people see them as different. It's all in the details of the response curves of the receptors, and how the brain process the information it gets from them. I see what I consider to be vivid colors, but there are certain shades of red and green that I cannot distinguish. I have absolutely no problem telling you when my computer screen is shining all blue or all red or all green. Some people may not even be able to do that.

The eye/brain can be tricked into thinking it is looking as some color it is not even seeing by stimulating the receptors with the right combination of other colors. The receptors only sense intensity. They do not know what part of their response curve is being activated. For any individual, there are many different combinations of wavelength intensities that will be perceived as the same color. It just depeds on the shape of their response curves. A proper balance of red, green, and blue can fool you into thinking you are seeing yellow or white. Does it have to be exactly the same monochromatic red, and blue, and green to accomplish this? Absolutely not. Take any red and any blue and any green and you can adjust the intensities to make you perceive yellow. So we call red and green and blue the primary colors, and use them in our display devices to produce all kinds of color. Enough said about this. You can find out lots about it on the internet.

The color of copper is what we perceive because of the mix of intensities of whatever colors it reflects. It probably does reflect every color that hits it, but some more efficiently than others. What we percieve as white light hitting the copper has its intensity profile altered when it is reflected to that blend of colors we call- what else- copper. Is it the same for me as it is for you? Probably not. But everytime I see copper I recognize the color as copper and everytime you see copper you recognize it as copper, so we both agree- the color of copper is copper.

The point is, even subtle variations in the relative intensities of reflected light can dramatically change our perception. You don't have to have absorption spikes or localized peaks to alter the perceived color of a broad mix of wavelegths like you expect from a metal. A slight lowering of the blue end and elevation of the red end will change your perception considerably. If you had the time, you could fool around with your computer screen to see how much your color perception changes as you change the levels of RGB. When you consider how broad the response curves of your receptors are, you should not be surprised how easy it is to alter your color perception with relatively small distortions of the spectra we perceive as white. What is truly amazing is how much color variation we perceive from such a limited set of sensors.

 

[quasar987]

Selective reflectivity, huh? How does that happen, what's the cause? Is it because of the index of refraction's dependence on frequency, some colors are filtered out?

 

[OlderDan]

Selective reflectivity, huh? How does that happen, what's the cause? Is it because of the index of refraction's dependence on frequency, some colors are filtered out?

The absorption mechanisms are complex. The electron energy states in conductors and semiconductors form continuous energy bands with gaps in between. Photons can be absorbed by electron transitions across gaps, with some in the visible range. This is complicated stuff! There is a lot more going on in a metal than the response of the conduction electrons to an impinging EM wave. The photoelectric effect is one example. If you want to understand the mechanisms, you will need to study some solid-state physics to understand the electron energies within solids and their interactions with photons.

Index of refraction is just a measure of the speed of light in a substance. It is a factor in determining the transmission of light through a substance and reflection of light at the surface, but it has nothing to do with how much light is absorbed. The speed reduction is also due to complex interactions btween the EM wave and the charged particles in the substance. That is different from the absorption mechanisms that account for the colors of objects that reflect or transmit light.

I'm not going any further with this because 1) it is a huge subject and 2) I've not looked at any of this for a loooooooong time. Some of the younger more current folk can no doubt do more with it, but you not going to get a simple answer.

 

[quasar987]

Alright, thanks a lot Dan!

My idea regarding index of refraction was that the reflection and transmission coefficient when a wave changes medium is dependant on the ndex of refraction. So since it is dependant of frequency, maybe, the reflection coefficient would be near 0 for some frequency. This mechanism would then act as a "color absorber".

 

[Claude Bile]

Index of refraction is just a measure of the speed of light in a substance. It is a factor in determining the transmission of light through a substance and reflection of light at the surface, but it has nothing to do with how much light is absorbed.

I wouldn't say that the refractive index and absorption are completely distinct, they are connected via the Kramers-Kronig relations. Absorption peaks have a strong influence on the refractive index for nearby wavelengths.

Claude.

 

[OlderDan]

I wouldn't say that the refractive index and absorption are completely distinct, they are connected via the Kramers-Kronig relations. Absorption peaks have a strong influence on the refractive index for nearby wavelengths.

Claude.

Fair enough. If someone wants to pursue that train of thought, its all yours.

 

[Claude Bile]

Quasar started a thread on the KK relations in the classic forums I think, so I'll let him (her?) take the lead on this one .

Claude.

 

[ryanbreslin]

If blue paper is blue because it aborbs wavelengths other than blue and reflects blue back to us, where does the energy of the absorbed wavelengths go? Surely it can't just keep being absorbed by the material in question and the electrons must drop to lower energy levels again and emit the absorbed energy? Wouldn't that mean the paper would essentially be reflecting white light and would therefore look white?