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Where does colour come from?

  1. Oct 4, 2013 #1
    Hi, I'm a recent high school graduate and I was just wondering about where colour comes from. I know about the flame tests of different metals and that the sky is blue due to refraction and stuff, but what about just everyday objects? Is wood brown because the electrons in the wood particles (I guess carbon, nitrogen and stuff) absorb a certain frequency of light and emit it back, just like flame tests? It just confuses me.
  2. jcsd
  3. Oct 4, 2013 #2


    Staff: Mentor

  4. Oct 4, 2013 #3
    Thanks, but I knew all of that already. But what does it mean that the objects reflect light? How does it exactly "differentiate" between the different wavelengths? For atoms, there are the different energy levels that each correspond to a frequency, but how does that work in different objects that are made of more than one element?
  5. Oct 4, 2013 #4
    different materials have different levels of energy for electrons , if the frequency of a specific colour is equal to the that of the electron , the electron absorbs that color , gets excited to higher level , then releases the same frequency * colour * wave and going down a level ( could be a level , two , it depends on the electron ) the rest of wavelengths are absorbed by the atom and transformed into heat energy
  6. Oct 4, 2013 #5

    Claude Bile

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    First things first.

    What you interpret as colour is a result of signals coming from receptors in your eyes. Each type of receptor (red, green, blue) is excited by a different amount from different frequencies, thus allowing your brain to differentiate between frequencies by combining signals from the different receptors to perceive colour.

    The proper radiometric (i.e. eye and brain-independent) way to pose questions pertaining to colour is through the concept of a spectrum. A spectrum defines the energy as a function of frequency of a given optical (or electromagnetic signal).

    Thus when we say an object is brown - what we really mean is that the frequency content of the light waves is such that we perceive it to be brown based on the signals from the colour receptors in our eyes. If we were to measure the spectrum directly (say, using a spectrometer), we would instead get a curve that showed energy versus frequency.

    Now the question becomes, why do objects reflect a certain spectrum? This depends on the properties of the object.

    Emitted light is easy to study, because it depends ONLY on the object - this is why the sun has the colour that it does, and why metals in flame tests emit the colours they do. Reflected light however depends on both the object AND the manner in which it is illuminated. For objects that do not emit their own light, the spectrum we see is due to a combination of the illumination spectrum and reflection spectrum.

    So what determines the reflection spectrum of an object? For bulk solids like wood, it is due to the characteristics of the chemical bonds in the material, and not just the elemental constituents themselves. Consider the example of diamond and graphite - both are comprised of pure carbon. They exhibit different optical properties because the carbon-carbon bonds have different characteristics in each solid.

    In short, the colour of something reflects the atomic and molecular nature of the bonds that hold the object together - this is why spectroscopy is such a powerful tool when it comes to analysing materials (see for example http://en.wikipedia.org/wiki/Raman_spectroscopy).

  7. Oct 4, 2013 #6


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    nice answer there. also, I'm happy that people seem to be using the spelling colour. I thought most people on PF were American?
  8. Oct 4, 2013 #7


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    I'm going to spell it as "color".

    I think this is a very good place to introduce one of the most common research instrument and technique. I think people need to know that we can use the transmission and absorption of light from a material to study the properties of the material. Therefore, we know quite a bit on what caused certainly properties, such as colors.

    If you've never heard of it, look at something called UV-VIS spectroscopy. This document describes the basic principle of this technique:

    http://www.plant.uoguelph.ca/research/homepages/raizada/Equipment/RaizadaWeb%20Equipment%20PDFs/5B.%20UV%20VIS%20theory%20ThermoSpectric.pdf [Broken]

    In relation to the question asked in this thread, check out Pages 6-8 that clearly explains the mechanism on why we see the color that we see from different objects. Note that the principle mechanism here is due to the vibrational modes/states of molecules or solids. These are NOT atomic transitions, contrary to popular myths. Such vibrational modes are not present in isolated, free atoms or atomic gas.

    This UV-VIS technique is similar to the Raman spectroscopy mentioned earlier, but with less complication because it normally does not use polarized light source. I've seen this instrument being used as part of an advanced undergraduate physics lab course. So besides being used for research, it is also a very useful educational tool.

    Last edited by a moderator: May 6, 2017
  9. Oct 6, 2013 #8


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  10. Oct 8, 2013 #9


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    Great explanation of why glowing iron isn't "laser-pure". Saying they are not atomic transitions is pretty broad though since pure Iron in a rod exhibits atomic behavior (it's crystaline, not molecular, I think). I'm not familiar with the popular myth of "atomic transitions". They definitely are not electronic transitions only, which I thought was the popular myth. (be gentle)
    Last edited by a moderator: May 6, 2017
  11. Oct 8, 2013 #10


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