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Thread: Black/White and energy absorbtion

  1. #1 Black/White and energy absorbtion 
    Forum Isotope (In)Sanity's Avatar
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    Ok, I realize I can find this on the net. I was at a home and garden show here in the valley and came across a guy selling what appeared to be almost a white corn starch to pain your home with that was suppose to keep things cool. He had a heat lamp setup shining down on two pieces of plate steel. One was painted with this white stuff, and the other was not. The two pieces were very dark (almost black) in color. Now please correct me if I'm wrong, but doesn't anything painted white reflect light and some heat? His painted white sample was not nearly as hot as his black sample, but still clearly warm.

    I know a mirrorized surface will reflect light and heat, and a vacuum sandwich will do even better.

    So does anything painted white help reflect heat, or just light?


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  3. #2  
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    It's more accurate to say that the light has rays of many different wavelengths (thus, colors), and the white stuff/mirrorized surface reflects many of them, especially those in the visible spectrum.


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  4. #3  
    Forum Isotope (In)Sanity's Avatar
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    Quote Originally Posted by Zero
    It's more accurate to say that the light has rays of many different wavelengths (thus, colors), and the white stuff/mirrorized surface reflects many of them, especially those in the visible spectrum.
    Yes, but what about heat? Heat of course being a form of radiated energy and sharing close proximity to light in the spectrum.
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  5. #4  
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    Perhaps I'm incredibly stupid, but heat=light. The heat, the release of energy, is discharge of energy when photons hit the surface of your skin.

    Mr U
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  6. #5  
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    Heat is not a wave. It's silly to think of heat traveling around.

    The energy in the electromagnetic waves, including the visible light, is transferred to object N when N absorbs the waves. This energy transfer raises N's temperature.

    I think that's what you're looking for.
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  7. #6  
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    Quote Originally Posted by HomoUniversalis
    Perhaps I'm incredibly stupid, but heat=light. The heat, the release of energy, is discharge of energy when photons hit the surface of your skin.

    Mr U
    No, Universalis is right. The sun's rays are heat are they not? And they are warm are they not? And so wouldn't white paint, which reflects light, not also reflect heat waves too, would it not? And how annoying me phrasing my answer in the form of rhetorical questions?
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  8. #7  
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    Quote Originally Posted by Locke
    No, Universalis is right. The sun's rays are heat are they not? And they are warm are they not?
    No, the sun's rays are electromagnetic waves (light). "Heat" is the energy of the thermal motion of particles in a substance. When a substance is hot, the particles vibrate faster. This faster motion requires energy, and that energy is what we call "heat". When light strikes a material some the light is absorbed, and some of the energy of that absorbed light will go toward making the particles in the material move faster.

    Infra-red light, which is right below visible light on the electromagnetic spectrum, is no different from any other kind of light; it just has a wavelength that's too long for our eyes to be able to detect it.
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  9. #8 light 
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    nearly all correct. Heat is a confusing concept here because it is only the way we sense vibration of particles. Vibrational energy is associated with infra-red radiation. Visible light and UV are both associated with electric energy which is does not cause vibration and is not sensed as heat. All materials have specific absorbtion/emission spectrums, and so it really depends on the material. Metals are notoriously bad at storing electrical and vibrational energy, so that they readily give these forms up (hence good conductance) and when you touch a metal it will readily give up its heat energy to your hand and will feel hot. The oposite is true for water which can hold a lot of vibrational energy and often feels cool. Overall it depends what the specific energy attributes are of the coating substance as to how well it will insulate the metal sheet.
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