In our post Part 1: Why Does Plastic Yellow, we mentioned chromophores. This post is intended to dig a little dipper into what they are and how they work. So here it goes!
Chromophores are any part of the function group (read molecule) with a large number pi-electronics like conjugated double bonds (or triples bonds, but generally not in single bonds). The more bonds greater the wavelength, the fewer the bonds, the shorter the wavelength. These bonds can change over time. And when they do they can be referred to as a bathochromic shift (more) or hypsochromic shift (less bonds).
So the arrangement of the chromophore pi-bonds determines the color of an object.
It's worth taking a minute to discuss the visible UV spectrum here as well. You may recall the picture on this post of a prism breaking up light. That picture is awesome. Not only because it makes an excellent album cover, but because it demonstrates that white light is made up of all the colors of the visible spectrum. When light hits an object, it absorbs some, and reflects the others. This is due to chromophores within the object (and those pi-bonds we mentioned) and it is why some objects appear red, some blue, and some yellow. However, as chromophores change, the object can take on other colors. They can change through many different ways -- and that's the subject of Part 3!
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Part 1: Why Plastics Yellow: Intro/Summary
Part 3: Why Plastics Yellow: Mechanisms of Degradation
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