My 7yo snapped the clip off of a transparent pink plastic pen. He noticed that at the place where it broke, the transparent pink plastic became opaque white. Why does that happen (instead of it remaining transparent throughout)?
This is best illustrated by the pic I took of the [broken pen](https://imgur.com/S8rasqb).
In: Physics
Yay finally my time to shine!
Plastics are made of polymers, which are long molecules, all entangled together – imagine cooked spaghetti. In this state, the material is see-through. This is known as amorphous, and is the reason glass is see-through too.
When you bend the plastic, you stress these polymer chains and stretch them out. This allows them to align together, imagine raw spaghetti. In this state, the polymer chains can crystallise, and this blocks light.
Crystallisation is essentially just the process of creating an ordered structure of atoms or molecules.
To prove this, try heating the plastic up a bit, and see if it goes transparent again. The heat allows the chains to move back into their relaxed position.
Source: have a degree in Materials Science.
EDIT: Seems most of these other answers are contradictory, shows how misinformation can spread. Best is to just read up yourself: https://www.polymersolutions.com/blog/why-does-plastic-turn-white-stress/
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