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A question about colours

  1. May 23, 2006 #1
    If I look at for example my curtain I see that it is green. But why does it have this colour? I know it has something to do with all colours except the green being absorbed but the curtain is just a pile of molecules. So how come this pile has colours? and how come not all substances have(for example glass doesn't have a colour)?


    P.S. could the person answering this please explain it in plain, easy english because I'm just 15 years old and I live in the Netherlands.
  2. jcsd
  3. May 23, 2006 #2
    The notion of colour is an invention of the human brain. The molecules in the curtain are complicated objects that can absorb and emit electromagnetic (EM) radiation at specific wavelengths. What happens is that in the region of the EM spectrum that the human eye can perceive (called visible light), all wavelengths are absorbed and only the ones that are specific to the molecules are emitted (so the curtain emits wavelengths that make up the colour "green" in your brain).

    The human eye then picks up these wavelengths (in fact they are absorbed by cells in the eye called retinal cells). There are two kinds of cells, rods and cones, where one picks up "brightness" and the other picks up "colour". There are 3 kinds of colour-recognising retinal cells, which absorb colours roughly red, green and blue. This is why these are the primary additive colours, and every colour the human eye perceives can be composed of these three colours, with different amounts of the primary colours for a different combined colour. The human brain picks up the response from the retinal cells and then analyses the data to give humans the sensation of vision.
    Last edited: May 23, 2006
  4. May 23, 2006 #3
    Thanks, I think I understand now.
  5. May 23, 2006 #4
    I think it's kinda weird to think that colour doesn't really exist. While other things like heat are "derived effects", colour is just so totally imaginary.
  6. May 24, 2006 #5
    Just one more question. Those "complicated objects", are they built up of more than one molecule? And, if so, how many molecules are needed to form an object with colour?

  7. May 24, 2006 #6
    Any atomic system absorbs and emits EM radiation at specific wavelengths. So any atomic system has a spectrum: this spectrum is the wavelengths of EM radiation it emits.

    The simplest of such systems is the hydrogen atom: just one proton and one electron. If you go on to study physics, this will be one of the first real problems you will tackle in your quantum mechanics course.

    The curtain, however, is made up lots of different atoms all connected together, and this is the reason I called it a complicated object.
  8. May 24, 2006 #7
    So, a single "curtain molecule" absorbs all light and emits "green light".
    Thanks, now I really understand.
  9. May 24, 2006 #8


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    The curtain is dyed green. Without dye, the curtain's would be probably be white or off-white. (In fact, they almost assuredly bleached all the colour out of the curtain material so the dye would "take", unimpeded).

    Anyway, the point is that, especially in textiles, colour is determined by dyes. Dyes are carefully constructed molecules (though often they are natural) chosen specifically for the purity and "colourfastness"* of the colour. (Colourfastness refers to the stability of the colour over time and weathering, most notably, fading due to exposure to bright sunlight.)

    The science of colour dyes has a very long and venerable history.

    Many dyes are actually natural minerals, and some are directly from elements, such as cobalt (blue), chromium (white), cadmium (yellow or red).

    Webpage (light reading): A History of Colours
    Book (heavy reading): Colour: Travels Through the Paintbox
    Last edited: May 24, 2006
  10. May 24, 2006 #9
    Yes, although for all extents and purposes, a molecule of the curtain really refers to a molecule of the dyed curtain.
  11. May 24, 2006 #10
    further on the topic of light, i understand certain elements have distinct spectra due to the quantized energy levels and so on, but why doesnt this happen with solids? why does a hot solid emit a continuous range of frequencies as it is heated, but a gas exhibits the quantized frequencies?
  12. May 24, 2006 #11
    A solid (or even better a blackbody) has more modes of vibration (read this as energy levels), essentially because it's a more complicated system. Therefore the frequencies of radiation it can emit, which is constrained to the differences between these energy levels, is greater.
  13. May 25, 2006 #12
    So the radiation emitted in blackbody radiation is not the change in electrons' energy levels, rather in the individual atoms or molecules? and if so, how does that generate photons?
  14. May 25, 2006 #13
    A blackbody is an idealized system, but it has infinite modes of vibration. If you have solved the energy eigenvalue equation of the hydrogen atom or the harmonic oscillator in QM, you will know that particles have energy levels. If you now consider a huge Hamiltonian, consisting of the kinetic and interaction energies of thousands upon thousands of particles, you will realise that there is likely to be many more modes of vibration. Also do not forget that the electron of one atom can interact (and will interact) with the electron of another, so you cannot truly speak of an "individual atom or molecule."

    Any change in electrical potential energy will emit/absorb a photon; it does not necessarily have to be a change in the energy level of an electron.
  15. May 26, 2006 #14


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    I think we should let the OP come to his own conclusions about understanding.

    If you examine his posts:

    "...the curtain is just a pile of molecules. So how come this pile has colours? and how come not all substances have [colour]..."

    "...Those 'complicated objects', are they built up of more than one molecule? And, if so, how many molecules are needed to form an object with colour?..."

    "...So, a single 'curtain molecule' absorbs all light and emits "green light"..."

    you'll see that an understanding of curtain molecules vs. dye molecules will surely help him better grasp the processes here.
  16. May 26, 2006 #15
    You make a good point.
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