eli5 Why do colors fade on things left in the sun?


eli5 Why do colors fade on things left in the sun?

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Things have colors because of how the electrons are arranged in the molecules. When UV rays from the sun hit those molecules, it can break them apart which changes how the electrons are arranged, meaning they lose their color.

Pigments are designed to absorb all of the light except the color you want.

UV light is VERY high energy.

When (certain) UV light is absorbed, it tends to break down whatever absorbed it. Pigments. Plastics. DNA. You name it.

So, as the pigments are bombarded with UV radiation over time, the pigments break down and stop absorbing the frequencies of light you don’t want to see. The more of the spectrum it reflects, the more “faded” it appears as it starts to reflect more than just the color you want reflected!

A coloured thing is coloured because it reflects certain wavelengths of light and absorbs the rest. All photons of light carry energy so the energy of the photons that get absorbed has to go somewhere. Mostly it gets degraded into heat but the chemical structure of some dyes are mote fragile than others. Absorbing the photon can cause it to break apart and lose its colour. Typically, colours that are based on metal compounds are stable, for instance the green or blue of copper salts or the red and ochre of iron oxides. Those depend on the electronic structure of the metal. By contrast, dyes based on organic, carbon compounds get their colour from the particular way certain atoms are joined together. If the photon breaks a bond, the structure changes and the vast majority of organic compounds aren’t highly coloured.

When we colour things we use pigments. These pigments can often (not always) be broken down by UV light. The sun emits lots of UV light. The majority of it is filtered out by our atmosphere but enough still gets through that it can slowly damage the molecular structure of the pigments. Eventually the molecules break down altogether and that’s when colour fading or sun bleaching occurs.