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If you leave something outside, its colors seem to inevitably fade or bleach due to exposure. Is this due to UV absorption? What sort of mechanism causes this - is it that man-made dyes deform on a molecular level? Are there notable materials that are exceptions?

To turn this string of questions into a focused query, let me refer back to the title:

Why does sunlight cause colors to fade?

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This is more chemistry than physics... The color of a material is due to an interaction of the light with chemical bonds (usually double bonds) in the dye. The UV component of sunlight tends to knock electrons out of double bonds and can in time cause changes to the chemical composition which we experience as bleaching.

For a more thorough and authoritative answer I would recommend asking this question on the chemistry stack exchange.

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  • $\begingroup$ Thanks for the tipoff about Chem SE. Here's what some of their folk had to say: chemistry.stackexchange.com/questions/29445/… $\endgroup$
    – zahbaz
    Commented Apr 30, 2015 at 3:29
  • $\begingroup$ @zahbaz good to see that the essence of my answer was correct although you did get a more complete answer from the Chem.SE - thanks a lot for posting that link! $\endgroup$
    – Floris
    Commented Apr 30, 2015 at 3:44

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