# Why does the sun bleach colors?
**URL:** https://boards.straightdope.com/t/why-does-the-sun-bleach-colors/488021
**Category:** Factual Questions
**Created:** [March 2, 2009, 4:24pm UTC](https://boards.straightdope.com/t/why-does-the-sun-bleach-colors/488021 "2009-03-02T16:24:39Z")
**Posts on this page:** 13
**Page:** 1
### Author:  [@YogSothoth](https://boards.straightdope.com/u/YogSothoth)
#### Post date: [March 2, 2009, 4:24pm UTC](https://boards.straightdope.com/t/why-does-the-sun-bleach-colors/488021/1 "2009-03-02T16:24:39Z")
I have a beanie snowman sitting on my car’s dashboard. Its been there for more than 5 years. It used to have a black hat, with a red and green scarf. However, now the hat is gray, the green is gray, and the red is like a faded magenta.
1. Why does the sun make colors fade?
2. Why didnt the red fade to gray too? Is that spectrum just not as susceptible? Does it have something to do with the longer wavelength?
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### Author:  [@cjepson](https://boards.straightdope.com/u/cjepson)
#### Post date: [March 2, 2009, 4:49pm UTC](https://boards.straightdope.com/t/why-does-the-sun-bleach-colors/488021/2 "2009-03-02T16:49:28Z")
The sun’s energy breaks down the chemicals that make the pigments. The extent of bleaching and the ultimate color would depend on the particular properties of the chemical involved. Beyond that, IDK (IANAChemist).
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### Author:  [@Solfy](https://boards.straightdope.com/u/Solfy)
#### Post date: [March 2, 2009, 5:16pm UTC](https://boards.straightdope.com/t/why-does-the-sun-bleach-colors/488021/3 "2009-03-02T17:16:03Z")
IAA chemist. A dye chemist, as luck would have it.
\*\*cjepson \*\*is right. The UV radiation from sunlight breaks bonds in dye molecules. These changes can result in a previously colored molecule either no longer giving color, or no longer giving the desired color. Different molecules are used for different colored dyes, and they all break down differently. In some cases you have a loss of intensity, in others you may see yellowing. If the colored item was made up of a combination of dyes, you may see one dye fading while the remaining color remains stable which would result in an apparent color shift.
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### Author:  [@YogSothoth](https://boards.straightdope.com/u/YogSothoth)
#### Post date: [March 2, 2009, 5:21pm UTC](https://boards.straightdope.com/t/why-does-the-sun-bleach-colors/488021/4 "2009-03-02T17:21:34Z")
> [@Solfy](#):
>
> IAA chemist. A dye chemist, as luck would have it.
> \*\*cjepson \*\*is right. The UV radiation from sunlight breaks bonds in dye molecules. These changes can result in a previously colored molecule either no longer giving color, or no longer giving the desired color. Different molecules are used for different colored dyes, and they all break down differently. In some cases you have a loss of intensity, in others you may see yellowing. If the colored item was made up of a combination of dyes, you may see one dye fading while the remaining color remains stable which would result in an apparent color shift.
Thanks **Solfy**. That brings me to another question I just thought of. Since white light is all colors, and black is the absence of color, why doesnt it fade to black? It doesnt seem to make sense that the snowman’s fading to a lighter shade with presumably more colors
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### Author:  [@KlondikeGeoff](https://boards.straightdope.com/u/KlondikeGeoff)
#### Post date: [March 2, 2009, 5:29pm UTC](https://boards.straightdope.com/t/why-does-the-sun-bleach-colors/488021/5 "2009-03-02T17:29:31Z")
Depending on the material, some colors also fade or chage faster than others. In color photographic prints, for example, red seems to fade the fastest, or change to magentia. We use dto call those “fugitive colors.”
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### Author:  [@CJJ](https://boards.straightdope.com/u/CJJ)
#### Post date: [March 2, 2009, 5:47pm UTC](https://boards.straightdope.com/t/why-does-the-sun-bleach-colors/488021/6 "2009-03-02T17:47:27Z")
> [@YogSosoth](#):
>
> Thanks **Solfy**. That brings me to another question I just thought of. Since white light is all colors, and black is the absence of color, why doesnt it fade to black? It doesnt seem to make sense that the snowman’s fading to a lighter shade with presumably more colors
You are confusing the [additive](http://en.wikipedia.org/wiki/Additive_color) color mixture of light with the [subtractive](http://en.wikipedia.org/wiki/Subtractive_color) color mixture of pigments. Blue (more precicely cyan) and yellow pigments, for example, mix to green because the two colors destructively interfere with the light reflected by the pigment. But shine a blue and yellow light on the same spot and the two additively create white.
If one of the pigments in a mix breaks down, the dye will therefore get lighter in color because there is one less pigment “subtracting out” the reflected light. Conversely, if you lose part of the color spectrum in a light source, the light becomes darker.
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### Author:  [@Solfy](https://boards.straightdope.com/u/Solfy)
#### Post date: [March 2, 2009, 6:00pm UTC](https://boards.straightdope.com/t/why-does-the-sun-bleach-colors/488021/7 "2009-03-02T18:00:48Z")
> [@YogSosoth](#):
>
> Thanks **Solfy**. That brings me to another question I just thought of. Since white light is all colors, and black is the absence of color, why doesnt it fade to black? It doesnt seem to make sense that the snowman’s fading to a lighter shade with presumably more colors
It’s not quite as simple as white being all colors and black being no colors.
For an opaque object, white is the reflectance of all colors. Black is what you get when all the colors are absorbed.
A green snowman scarf is absorbing most of the light that hits it but reflecting (technically scattering) the green portion of the light back to your eyes. The black snowman hat was absorbing all the colors of light that hit it, but now that they dyes have faded, more of the light is being scattered back to you, making it appear gray.
For something to “fade” darker, it would have to absorb more color. Broken molecules tend to lose their ability to absorb color.
There’s a nice basic explanation of the phenomenon [here](http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/light/u12l2c.html).
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### Author:  [@Superfluous\_Parentheses](https://boards.straightdope.com/u/Superfluous_Parentheses)
#### Post date: [March 2, 2009, 6:04pm UTC](https://boards.straightdope.com/t/why-does-the-sun-bleach-colors/488021/8 "2009-03-02T18:04:59Z")
> [@YogSosoth](#):
>
> Thanks **Solfy**. That brings me to another question I just thought of. Since white light is all colors, and black is the absence of color, why doesnt it fade to black? It doesnt seem to make sense that the snowman’s fading to a lighter shade with presumably more colors
White **light** is “all colors”, but materials have color because they _absorb_ the (white) light and _reflect_ certain wavelengths (colors) better than others. Pigments that break down don’t necessarily break down to molecules that don’t reflect at all - it’s more likely that they break down to some other color (but probably a color that isn’t as pronounced). Also, the pigments in the dye probably break down faster than the base material, meaning the base material’s color will shine through.
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### Author:  [@runcible\_spoon](https://boards.straightdope.com/u/runcible_spoon)
#### Post date: [March 3, 2009, 2:02am UTC](https://boards.straightdope.com/t/why-does-the-sun-bleach-colors/488021/9 "2009-03-03T02:02:58Z")
Are there dyes/paints that will break down into more saturated versions? Or is there something inherent in the breaking of molecules that makes them tend towards desaturation? Or, come to think of it, is it just that if there was a readily available super-saturated version, they’d just use that and dilute it?
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### Author:  [@WarmNPrickly](https://boards.straightdope.com/u/WarmNPrickly)
#### Post date: [March 3, 2009, 2:44am UTC](https://boards.straightdope.com/t/why-does-the-sun-bleach-colors/488021/10 "2009-03-03T02:44:04Z")
> [@Solfy](#):
>
> …Broken molecules tend to lose their ability to absorb color…
This is the biggest part of it. Sunilght often causes paired electrons to split. Once they split, it’s a gamble if they will recombine to they way they were originally arranged. Dyes almost always have heteroatoms (usually Nitrogen or Oxygen, but heteroatom means not carbon in this case). Heteroatoms increase the tendency for these types of reactions.
The only specific reaction I know off the top of my head are the [Norrish Type](http://en.wikipedia.org/wiki/Norrish_reaction) reactions. You can pretty much substitute N,P, or S for O to get a similar reaction. This is a simple system though and dyes will not be simple because of the large interconnected electron system. When electrons in a dye split, they could be at opposite ends of the molecule as fast as Heisenberg knows their energy.
> [@](#):
>
> Are there dyes/paints that will break down into more saturated versions? Or is there something inherent in the breaking of molecules that makes them tend towards desaturation? Or, come to think of it, is it just that if there was a readily available super-saturated version, they’d just use that and dilute it?
Wouldn’t that be cool? If I knew of such a thing, I would coat some paper with it then tape it to the back of my camera obscura. Before you know it I would have some kind of picture.[/sarcasm]
Things that behave this way tend not to derive their color from large conjugated systems, instead they are metallic based. Silver compounds behave this way, but I’m sure there are other metallic compounds that get their color this way. Organic compounds nead to be put in large chains to lower the HOMO-LUMO gap. Metals already have a low HOMO-LUMO gap.
[HOMO-LUMO](http://en.wikipedia.org/wiki/HOMO/LUMO)
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### Author:  [@runcible\_spoon](https://boards.straightdope.com/u/runcible_spoon)
#### Post date: [March 3, 2009, 9:58am UTC](https://boards.straightdope.com/t/why-does-the-sun-bleach-colors/488021/11 "2009-03-03T09:58:07Z")
> [@WarmNPrickly](#):
>
> Wouldn’t that be cool? If I knew of such a thing, I would coat some paper with it then tape it to the back of my camera obscura. Before you know it I would have some kind of picture.[/sarcasm]
:smack:
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### Author:  [@BACI](https://boards.straightdope.com/u/BACI)
#### Post date: [March 3, 2009, 10:59am UTC](https://boards.straightdope.com/t/why-does-the-sun-bleach-colors/488021/12 "2009-03-03T10:59:31Z")
I always thought things faded because they got filled up with photons. Which are of course white.
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### Author:  [@Solfy](https://boards.straightdope.com/u/Solfy)
#### Post date: [March 3, 2009, 2:06pm UTC](https://boards.straightdope.com/t/why-does-the-sun-bleach-colors/488021/13 "2009-03-03T14:06:34Z")
> [@runcible\_spoon](#):
>
> Are there dyes/paints that will break down into more saturated versions? Or is there something inherent in the breaking of molecules that makes them tend towards desaturation? Or, come to think of it, is it just that if there was a readily available super-saturated version, they’d just use that and dilute it?
There are certain molecules (commonly called photochromic) that are colorless until activated by exposure to UV. The UV energy causes a reversible isomerization that switches the molecule into a colored state. In a system with any heat, once the UV energy is gone the molecule relaxes (or Transitions) back to its original uncolored state.
Such molecules would be very practical to have on one’s eyeglasses.