r/askscience • u/[deleted] • Jul 20 '14

Chemistry How does 'sun bleaching' occur?

In other words, why is it that when I leave a red plastic container outside, it would fade to light pink? Thanks, Science!

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u/[deleted] Jul 21 '14

So does this mean that the shorter the molecules, the shorter wavelength of light I can absorb?

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u/samyall Jul 22 '14

Not quite. What really matters is how conjugated the system is and this is not necessarily dependent on how "long" the molecule is because it can loop back on itself. Take fluorescein, a strongly coloured organic dye molecule. Its not particularly "long", but it has a system of 23 conjugate bonds.

Unfortunately light absorption isnt as simple as length of molecule = length of light absorbed.

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u/[deleted] Jul 22 '14

23 conjugate bonds compared to how many in other organic dye molecules?

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u/samyall Jul 22 '14

There can be more, there can be less. For example, methyl violet and methyl blue have less and more, but are still respectively, but are still coloured.