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  #1  
Old 08-15-2012, 02:39 PM
heebie heebie is offline
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Why does the color black attract more heat than the color white?

I hear wearing black makes you look much slimmer, but why is anything colored black more of a receptor of heat than things colored white? My father used to manufacture solar panels and we always painted them black; however, I never truly knew the science behind the fact. For example the car sun shield, black on inside, white/silver on the outside. Im 50, I gotta know the truth!!!
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  #2  
Old 08-15-2012, 02:41 PM
tdn tdn is offline
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White reflects light (and heat), black absorbs it.
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  #3  
Old 08-15-2012, 02:43 PM
Malacandra Malacandra is offline
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White reflects radiant energy better - that's why it's white, because it reflects all the colours of the spectrum. Black reflects none. I imagine that for the typical black and white pigments this also carries over into the infra-red, but of course if you absorb lots of radiant light it's going to warm you up anyway, because the energy doesn't just disappear. That's your quick and dirty answer anyhow.
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  #4  
Old 08-15-2012, 02:52 PM
Colophon Colophon is offline
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Yes, simple answer - dark-coloured things are dark because less light bounces off them. If less light is bouncing off them, it must be absorbed, so the object is absorbing energy.
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Old 08-15-2012, 03:28 PM
Punoqllads Punoqllads is offline
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There's one other step, that of blackbody radiation. Substances that absorb energy also emit energy. At the energy levels we're dealing with here, that's effectively heat energy.
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  #6  
Old 08-15-2012, 03:30 PM
Philster Philster is offline
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When you see white light, you are seeing all the colors of white light un-scattered. When you see a rainbow, you see white light that has been separated into its parts. Merged, it appears white... so it's white at full strength. Keep this in mind.

An object is mostly white if it reflects all the light (well, it is most of the light and is mostly white). The object itself is absorbing little light/energy but your eye is getting the bigger dose, thus the light appears white.

Very, very dark things (dare I say black) ain't giving up much light... they are absorbing that energy/light and heating up as a consequence.

Even very dark (black) objects reflect some light, but the point has been made: light colored objects look light because they reflect light/energy and the darker ones look dark because they absorb the light/energy. Your eye can only see what the object is giving up. If it gives up just green, you see green. If it gives up almost nothing, you see black.

If you eye sees very light objects, it does so because the light is getting into your eye and not the object. It's why a dark object doesn't blind you in the sun: It's not reflecting much light/energy.

.


.

Last edited by Philster; 08-15-2012 at 03:34 PM..
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  #7  
Old 08-15-2012, 05:09 PM
Chronos Chronos is offline
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Quote:
I imagine that for the typical black and white pigments this also carries over into the infra-red, but of course if you absorb lots of radiant light it's going to warm you up anyway, because the energy doesn't just disappear.
And in fact there's nothing special about infrared, either. We associate infrared with heat because most Earthly objects we consider warm or hot emit mostly in the infrared range, but that just says something about typical Earthly temperatures. The Sun is significantly hotter than most Earthly objects, and so heats up other things largely by virtue of its visible light.

Strictly speaking, if you wanted the "coolest color", you'd want something which is white in the visible range (so it reflects away most of the energy from the Sun), but black in the infrared range (so it'd radiate away as much of the heat it already has as possible). I don't know how feasible or practical it is to make dyes that behave this way, though.
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  #8  
Old 08-15-2012, 07:58 PM
Fiendish Astronaut Fiendish Astronaut is offline
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I'm unconvinced that white really is a cooler colour to wear. Has anyone actually proved it? Why do many desert tribes wear black?
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  #9  
Old 08-15-2012, 08:32 PM
Punoqllads Punoqllads is offline
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I would guess that for lightweight fabric, white cloth allows more light to pass through it than dark cloth. Plus, half of the heat the dark fabric radiates due to absorbing light will be away from the wearer.
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Old 08-15-2012, 08:41 PM
thelurkinghorror thelurkinghorror is online now
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Quote:
Originally Posted by Chronos View Post
And in fact there's nothing special about infrared, either. We associate infrared with heat because most Earthly objects we consider warm or hot emit mostly in the infrared range, but that just says something about typical Earthly temperatures. The Sun is significantly hotter than most Earthly objects, and so heats up other things largely by virtue of its visible light.
Yeah, the colors are not different "stuff" from UV, or IR, or for that matter radio waves, microwaves, and gamma rays. We consider the color part different because it's the only part that our visual systems are made to see.

Two points:
1) In the additive system that light uses, mixing colors eventually makes white. Black is the absence of light energy. This is the opposite of mixing paints, where they will eventually become black.

2) Every object is not really colored. It's just that each one is hit by white light, and it preferentially absorbs some colors, and the rest reflect off. Our visual systems then interpret that as color. So if we look at say, a frog, it will absorb mostly colors in the red and blue range, and the leftover green will bounce off and reach our eyes, and we will interpret green. If an object absorbs light poorly, it will bounce off all colors and we see white. If it absorbs, it will seem black. The light has energy, so the white object will reflect it towards us, while the black will absorb the energy and get warmer.

Quote:
Originally Posted by Fiendish Astronaut View Post
I'm unconvinced that white really is a cooler colour to wear. Has anyone actually proved it? Why do many desert tribes wear black?
I don't quite know, but isn't white the stereotypical color for sheiks and such? And indigo for the Tuaregs/Kel Tamasheq. I do know that they tend to not wear one robe-y thing, but have lots of thin layers, which helps with cooling. Something about keeping heat in the layers and away from skin?
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  #11  
Old 08-15-2012, 09:08 PM
Beware of Doug Beware of Doug is offline
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Could the wearing of black be a religious conviction of some kind?
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  #12  
Old 08-16-2012, 06:42 AM
Machine Elf Machine Elf is offline
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Quote:
Originally Posted by thelurkinghorror View Post
Every object is not really colored. It's just that each one is hit by white light, and it preferentially absorbs some colors, and the rest reflect off.
The absorption/reflection process you describe in detail there is typically reduced to the abstract "that object is <insert color>". To say that "every object is not really colored" is the same pedantic nonsense as when someone says "when you see something, you're not really seeing it; light is entering your eyeball and stimulating your retina, and your brain is interpreting the incoming signals as an image." Um, yeah, they've just described how a person sees.

Likewise, you haven't negated the idea that objects exhibit coloration; you've just explained the process by which they exhibit coloration. IOW, objects really do have colors.
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  #13  
Old 08-16-2012, 08:15 AM
hibernicus hibernicus is offline
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Quote:
Originally Posted by Fiendish Astronaut View Post
I'm unconvinced that white really is a cooler colour to wear. Has anyone actually proved it? Why do many desert tribes wear black?
The explanation I read somewhere (terrible cite, I know) is that the loose black robes of the Bedouin promote convective airflow inside the robe, which helps to keep them cool.

I'm not sure if that's correct though.
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  #14  
Old 08-16-2012, 08:52 AM
beowulff beowulff is offline
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Quote:
Originally Posted by Chronos View Post
Strictly speaking, if you wanted the "coolest color", you'd want something which is white in the visible range (so it reflects away most of the energy from the Sun), but black in the infrared range (so it'd radiate away as much of the heat it already has as possible). I don't know how feasible or practical it is to make dyes that behave this way, though.
This is called a "selective surface," and is generally done the other way (to maximize solar heat absorption).
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  #15  
Old 08-16-2012, 10:22 AM
TriPolar TriPolar is online now
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Black cloth can be more effective in intense sunlight because it blocks more light altogether. The white allows a certain amount to pass through. But I think in general, clothing colors have traditionally been more a product of the resources available for making cloth than a choice of preferred color. Also black looks cool, white just gets dirty.
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