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Is a large part of the energy of sunlight transmitted in the visible part of the spectrum? If not, then why would the saying that "bright colors reflect heat and dark colors absorb it" be true? Would it be due to a correlation between the reflecting properties of a material in non-visible and visible wavelengths?
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Somewhere near half of the energy is infrared. Two hypothetical substances that reflect equal amounts of visible light but different amounts of infrared would settle down at different equilibrium temperatures.Originally Posted by tashirosgt
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However, the idea of painting things (such as containers, roofs etc.) a light color to reflect the heat of the sun does work empirically. So I wonder if there is some property of the molecular structure of common materials that makes things that are a light color in the visible specturm also good reflectors of infrared light?
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If light-colored surfaces are consistently cooler, I would infer that the answer is yes.
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I think titanium dioxide is pretty reflective of infrared, and it's a common white pigment, so light colors are probably often full of it.
"Light colors reflect IR" might just be a good rule of thumb. I'm sure there are counter examples.
Edit: Oh, and carbon black absorbs IR, and that's in many dark colors. I recall some movement last year about California somehow forcing auto paints to reflect IR better, and carbon black, in all your nice dark colors, was a big problem.
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I think the point is that if something is a good reflector of infrared light, then it is likely a good reflector of visible light as well --- and therefore appears as a "light color".
Note that the colors we learn in grade school (red, purple, brown, black) all can be light as well --- we just call the light version a different names (pink, lavender, tan, white).
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Don't harmonics play into this?
I mean, if an object of a single color is emitting absorbed photons in a particular wavelength, then there are non-visible harmonics also emitted.
Therefore; something white, which is emitting in a an extremely broad spectrum of visible wavelengths would also be emitting an extremely broad spectrum in non-visible (ex. IR) wavelengths.
Does that sound right?
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No, harmonics are created a different way, they come from resonant cavities that resonate with a bunch of integer-multiple different frequencies. Molecules that emit light aren't resonant cavities, and have complex bands of frequencies rather than simple harmonics.
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these "colors" are wavelengths that we see, they are from 400 nanometers long to 1000 nanometers.
the surface that reflects them or absorbs them will interact with them (Photons) at the level that is same length as the photon striking the "surface" at that size.
so the surface architecture is what actually reflects and absorbs the light.
it would not surprise me at all if harmonic frequencies are creates in the Right circumstances say a surface with lots of regular ridges and crevices the same size of the wavelength directed at them.
although is this may be the same as destructive and constructive interference?
if this happens at multiple wavelengths, then harmonic secondary frequencies could be built.
off the top of my head, i am thinking about that iridescent blue butterfly with no pigment..
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i think the term for such a material is called a meta material..
http://en.wikipedia.org/wiki/Metamaterial
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http://www.physorg.com/news87144852.html
metamaterials are something different.
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That can be true, and when it is, you are right that you could imagine generating harmonics. It's not quite the same as the "resonant cavity" I was talking about, but it is pretty analogous in terms of the underlying physics.
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About then half the Suns energy is visible light, all other frequencies make up about the same amount. In terms of energy/Bandwidth visible light contains far more energy then anywhere else in the spectrum.
I’m not entirely sure bright colors is the same thing as how much energy gets reflected. Even in the visible spectrum the Sun doesn’t emit the same amount of energy in all bands. IIRC our eyes are more sensitive to certain colors generally correspond to the parts of the visible spectrum where the Sun emits the most energy.
How much light an object reflects is separate from it’s color. Color comes from the frequency of the light it reflects, not how much gets reflected.
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Nope, even if they were being generated the harmonics would be higher frequency emissions. IR is lower frequency than visible light.
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What if I stated it the other way around? In other words, a wide range of IR frequencies resulting in a brighter range of harmonics in the visible range.
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For the reverse, how likely, I think, is the question.
Heat Gain/Loss in Buildings (a recalcitrant PDF, Google html version was much easier to read)
Light or white paper? Light stone and brick? Blonde wood?Materials whose surfaces do not appreciably reflect infrared rays, for example, paper, asphalt, wood, glass and rock, have an absorption rate from 80% to 93%. Most materials used in building construction -- brick, stone, wood, paper, regardless of their color, absorb infrared radiation at a rate of about 90%.
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Yes, this is the reason white shirts are cooler on a sunny Summer day-- they emit infrared just as well as black shirts, but they absorb a lot less visible light. It's a reverse Greenhouse effect.
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Consider glare. Something can appear to be a dark color and still be reflecting a lot of light.
Also, fluorescence and phosphorescence come into play with some materials. For example, bright white paper has a brightness of 110, a percentage, because some of the UV energy that falls upon it is absorbed and then emitted as visible light. The same goes for most of the laundry detergents labeled as making "whites whiter and brights brighter". You can use a blacklight to see for yourself.
Et tu BAUT? Quantum mutatus ab illo.
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Are you saying that there is a molecule in the detergent that stays in the clothes and fluoresces the UV down into the visible?
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http://en.wikipedia.org/wiki/Fluorescent_brightener
Et tu BAUT? Quantum mutatus ab illo.
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There you have it, interesting.
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I'll have to stock up on that umbelliferone... I have guests coming this summer!
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