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Colour and heat

  1. Feb 8, 2004 #1
    Colour and heat!!

    Hello, im having a physics investigation about thermophysics stuff. I investigate how the rate of temperature increase inside a box alters by the colour of the box.. There are a few questions i want to ask:
    -a) Black box will abviously gets the hottest. But among red, blue, yellow, purple and green boxes, what order will it be?
    I was thinking that if heat transfer into a box is related to wavelength or frequency, red may get the hottest (as it has the shortest frequency) or..blue may get the hottest (due to the longest frequency) Or, heat transfer may not be related to wavelength or frequency. Im very not sure..
    b) How does the hot air get into the box? By molecules of the box material? Convection?
    c) If the distance between the heat source and the box increases, what kind of relationship will there be? and why does the box that is the furthest from the heater gets the least heat?

    Thank you soooo much for reading this and i hope you can write me back about them soon...
  2. jcsd
  3. Feb 8, 2004 #2
    Re: Colour and heat!!

    scratch that. Reverse it.
  4. Feb 8, 2004 #3
    wait, a blue box reflexs blue light (the strongest) and a red light reflexs red light (the weakest) so i think the red one would be hottest.
  5. Feb 8, 2004 #4
    Then there are "colors" not visible which affect the energy reflectivity/absorptivity of a customary non-blackbody.

    Heat travels directly from walls of box to air inside by conduction, radiation, convection - in that order of magnitude, I guess.
  6. Feb 8, 2004 #5
    i was referring to his frequency labelings. Blue light has the shortest, red light is the longest. as for what you're saying, yes you are probably right, because the red box absorbs the shortest highest energy wavelengths it would probably get warmer than the blue box. However as Loren said, other non-visible wavelengths could change that. There are other factors involved.
  7. Feb 8, 2004 #6
    Is the light spectrum (with red and blue) the reason why some people say radar guns (that cops use) seem to think red cars speed more?

    PS: I believe this is the 11000th post in this forum.
  8. Feb 9, 2004 #7
    well, my investigation results show the obvious results (eg. black heats up the quickest, most, and cools the quickest and silver and white heast up the least and stuff...) but among the blue, yellow, purple, red and green boxes, the results are quite random. Sometimes the green box heats up the most among the five and sometimes the red one does.. So i wanna get the theory right - is the bluebox supposed to get heated up first because it has more energy/higher frequency? if so, my investion must have got a lot of inaccurate and uncertain stuff (e.g. the temp might've got affected air conditioners or movement of people which mihgt've created wind effect...) But, if the theory suggests that the heat transfer is not related to wavelength/frequency, then my results are pretty right as they concluded that it's random (among the five colours). What do you people think??

    PS: thanks for your replies. those made me consider more about it
    PS: Congrats to Decker!
  9. Feb 9, 2004 #8
    I haven't heard such a thing. Radars use different wavelegths from visible light.
  10. Feb 9, 2004 #9


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    Colors are tricky. A box that looks yellow does not necessarily reflect only yellow light -- it might reflect equal parts of green and red light, and still look the same to our eyes (red light + green light looks exactly the same as pure yellow light).

    So, the actual amount of energy absorbed by a pigment is not just a function of its apparent color, but also its actual chemical composition and what frequencies it really absorbs. Not to mention the fact that sunlight has more energy in infrared photons than visible photons, and a pigment's visible appearance is not necessarily related to its infrared absorption...

    - Warren
  11. Feb 10, 2004 #10
    Sunlight offers whatever it collides with on the earth's surface with about 1000 Watts of radiant power per square meter.

    A 'good' black box will absorb most of that energy; a 'good' white paint (developed for that purpose) can scatter most of it away.

    Since most paints are designed not to absorb the maximum energy nor to reflect/scatter it, they will do something in between. I found a spec for the paint on road signs that specified that the white paint should reflect 85% of the incident light, and yellow 55%; I suspect these numbers refer only to visible light, adjusted for sensitivity of the human eye (lumens, not watts).

    You've already found that reflectance doesn't seem to correlate with the wavelength of the dominant color of the paint.

    If sunlight consisted of equal amounts of photons of all colors, I would expect a blue box to heat up the least--it would be turning away lots of high-energy photons.

    Since sunlight contains more green and greenish photons than it does red or blue, I might expect a green box to heat up the least--it's turning away many more photons than the other colors do.

    But as you've said, and Warren explained, there are reflections and absorptions of non-visible wavelength photons that mess up the results quite a bit.

  12. Feb 10, 2004 #11
    i thought it was UV and not IR that was hotter?
  13. Feb 11, 2004 #12


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    The Sun produces more IR radiation than UV radiation, I believe. UV is "hotter" in a sense because it is a shorter wavelength -- but the Sun produces less of it.

    - Warren
  14. Feb 11, 2004 #13


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    The shade of the color counts too. I have a dark blue car and it gets really hot when it sits out in the sun. A sky blue wouldn't get so hot.
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