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
Probably a combination of 2 things
1) The broken side isn’t smooth any more. Rough surfaces get hazy since they diffuse light that passes through them.
2) Plastic tends to bend before it breaks. When it bends, it’s ripping apart in a bunch of tiny cracks which traps air. These gaps and air bubbles diffuse light similar to reason #1
Source: I am a programmer and 3D artist who deals with surface materials and light transport equations for most of my day. Basically I get paid to ask the question “Why did the pen turn opaque when it broke,” and re-create it 😁
The break isn’t completely clean. The plastic probably has really tiny tears and cracks caused by the break which adds to the texture and diffuses light.
Plastic polymers are structures of relatively ordered chains of hydrocarbons.
When you put stress on this structure, you’re breaking apart molecular bonds in those chains, causing them to form small void spaces in the structure as they’re displaced and rearranged. These voids refract light differently than the rest of the structure, causing the opacity you observe.
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