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Colour and light absorption

  1. Feb 10, 2014 #1
    First, let me see if my facts are straight. White coloured objects reflect more light of ALL frequencies than black coloured objects, including infrared, etc. Hence a white coloured object is cooler than a black coloured object. If visible light was the only factor, a blue room would be cooler than a red room as blue light has more energy. I"m assuming same wall area and intensity of light. I wanted to know about the trends of these colours in infrared absorption. Does blue absorb less infrared than red? In a more realistic situation where the sun"s rays are incident on a wall, will a blue coloured wall be still be cooler now that other frequencies of light are also incident? Thanks.
  2. jcsd
  3. Feb 10, 2014 #2


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    The amount of IR light absorbed by an object does not depend on how well it absorbs visible frequencies. For example, a black trash bag is transparent to IR light.
  4. Feb 10, 2014 #3
    The fact that an object looks white (in visible light) does not guarantee that it will reflect well in infrared or any other frequency range.
    I don't think there is any trend. You just have to look at the specific absorption spectrum.
    An object may be blue (or red) for different reasons.

    The association between colors and "hot" and "cold" is not a physical one but rather psychological.

    Edit. I did not know that Drakkith is already on it. :)
  5. Feb 11, 2014 #4
    Thanks guys! So two objects that look exactly the same to us(say blue in colour) can absorb different amounts of IR radiation?
  6. Feb 11, 2014 #5


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    It's a good idea to avoid using the word 'colour' in any quantitative discussion in Physics except when the subject is 'perception' (and that won't be Physics). The eye is a really rubbish measuring instrument (spectrometer) and you cannot rely on it to tell you the right thing.
  7. Feb 11, 2014 #6


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  8. Feb 14, 2014 #7
    Ok great, thanks.
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