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I thought and read about color mixing today. I made some counterintuitive discoveries, and i now have some thought experiments which i cannot test.

I could print a yellow image but i use a different printing technique on each half. On the left side i use color which is true yellow(so yellow pigments which reflect yellow wavelength), on the right i use small red and green squares next to each other(not overlapping) which from afar appear also as the same yellow. So both sides appear yellow to me because my cone cells on my retina are tricked. if i am now holding a monochromatic yellow filter in front of me, the right side would appear black because red and green cant get trough the filter, and the left side would be ... still the same yellow. So i could now distinguish between the left and right side.

So my question is, what would i see when i took multiple monochromatic filters with me and look around? Are there materials which appear a specific color because they are(So that the atoms absorb every other color or just emits this specific color) and some materials which reflect or emit different wavelenghts which then appear as a specific color?

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closed as off-topic by user191954, G. Smith, JMac, The Photon, John Duffield Apr 30 at 17:25

  • This question does not appear to be about physics within the scope defined in the help center.
If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ I'm voting to close this question as off-topic because it appears to be about biology or human perceptions rather than physics. $\endgroup$ – The Photon Apr 30 at 16:20
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    $\begingroup$ @ThePhoton I disagree with your assessment. The question is not about human perception, while his direct question was "what would i see...", the core real question is his second question: 'Are there materials which appear a specific color because they are and some materials which reflect or emit different wavelenghts which then appear as a specific color?' A yes/no question to which I do not know the answer. $\endgroup$ – corcholatacolormarengo Apr 30 at 16:35
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    $\begingroup$ I also consider this question about physics; human eyes are sensors in this case. And the question is about reflectivity of substances $\endgroup$ – patta Apr 30 at 16:46
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    $\begingroup$ I also consider this question to belong here. $\endgroup$ – Helen Apr 30 at 17:36
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    $\begingroup$ @patta If you haven't already, you can nominate this question for reopening. $\endgroup$ – probably_someone May 2 at 11:49
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Every object has a spectrum of emitted light, which is a function $I(\lambda)$ that tells you the intensity $I$ of the light of wavelength $\lambda$ that is emitted by the object. Many different objects have many different spectra, and there are huge swaths of physics devoted to predicting, analyzing, and generally studying the spectra of various objects, from minerals to the atmosphere to stars and galaxies.

This makes it difficult to say what color an object is, because it's always emitting light composed of a bunch of different colors. That said, we can reasonably say that an object "appears a certain color because it is that color" when its spectrum is relatively sharply peaked in one color. A great example of this is a sodium vapor lamp, which gets its yellow color from the electrons in sodium atoms jumping between two energy levels. As such, its spectrum is very sharply peaked at 590 nm:

enter image description here

The above is an emission spectrum; the higher the points on the graph, the more intense the emitted light of that color is. For context, this is what the lamp looks like:

enter image description here

And for some more context, here are the spectra of a lot of other common light sources. Though they all look basically white, you can see that there are clear differences in their spectra that our eyes generally can't resolve:

enter image description here

In contrast, one of the green pigments in grass and most plants is chlorophyll b, which has a much broader and more complicated spectrum. Below I present the absorption spectrum, which tells you how much light is absorbed by the pigment. The convention is reversed: the higher the points on the graph, the more the light of that color is absorbed, so the parts of the graph that tell you the color that you see are the low points:

enter image description here

You can see from this graph that there are actually three component colors to chlorophyll b in the visible range: there's the obvious green color between 500-600 nm, there's a red portion past 650 nm, and there's a purple contribution from light lower than 450 nm. Your eyes take in this complicated red-green-purple mix and output "this looks green."

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  • $\begingroup$ Great detailed answer. Thank you very much. I feel kind of stupid because i knew that different objects have different spectrums but as i read about color mixing (additive and subtractive) i got kind of confused. It really was the first time that i connected it with, what would i actually see and how different your perception really is from reality. Thank you. $\endgroup$ – Sebastian Scholz Apr 30 at 20:43
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    $\begingroup$ @SebastianScholz BTW, the appearance of different spectra as the same color has its own name: metamerism. $\endgroup$ – Ruslan Apr 30 at 20:56
  • $\begingroup$ YES. Thank you. I so happy, that there is a word for this :D. Exactly what i was looking for. $\endgroup$ – Sebastian Scholz May 1 at 8:07
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You'd see a variety of results.

Imagine illuminating a material with white light, and looking at the spectrum of light it reflects. Any "colored" material will have peaks and valleys in its reflected spectrum. The same is true of materials transmitting or emitting light.

The human eye samples this spectrum in three bands, but those bands are wide, with lots of overlap. As a result, it's easy to come up with two materials that look like they're the same color, but actually have very different spectra. That's why we can see "full color" images on screens that have only red, green, and blue emitters. Your thought-experiment red-and-green checkerboard would be another example.

Does this happen in the real world? Are there materials that look yellow, but are really only emitting/transmitting/reflecting red and green light? Sure -- display screens, see above, as well as "multi-color" LED indicators. In the natural world, it may be harder to find examples, because most things are mostly grey-ish (equally reflective at all wavelengths) or brown-ish (more reflective at longer wavelengths). But if I had the filter you're describing, I'd head for a large garden and start inspecting all the yellow flowers; I'll bet I'd find something interesting.

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  • $\begingroup$ Thanks for your answere. I guess these monochromatic filters are hard to come by.Does something like this even exists? This would be interesting for photography. $\endgroup$ – Sebastian Scholz Apr 30 at 20:47
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    $\begingroup$ @SebastianScholz narrow band-pass filters do exist, see e.g. here. $\endgroup$ – Ruslan Apr 30 at 20:50
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    $\begingroup$ And thanks again :D $\endgroup$ – Sebastian Scholz May 1 at 8:19
  • $\begingroup$ @SebastianScholz Yep, solar astronomers live by them. The problem: if a filter blocks all but a very narrow band, it blocks most of the light, so you're left with a very dark image. Not so much of an issue if you're photographing the sun, but for anything else, you're talking very long exposures... $\endgroup$ – jeffB May 1 at 18:01
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The quick answer would be that there are indeed many objects with different spectra that appear yellow to us.

A spectrum is mapped onto just three values by the eye; if one for example consider the CIE color space the $(X,Y,Z)$ values are integrals over the spectrum $S(\lambda)$ multiplied by particular kernels $\bar{x}(\lambda),\bar{y}(\lambda),\bar{z}(\lambda)$. One can make different spectra with the same $(X,Y,Z)$ coordinates by tweaking the spectrum.

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  • $\begingroup$ Interesting representation, thanks i will read into that. $\endgroup$ – Sebastian Scholz May 1 at 8:01

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