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
Maybe a little more ELI5 to grow understanding:
Imagine a pane of glass. Light gets through glass because the glass is one solid piece with no discontinuities. When the glass cracks, maybe up close you could still see through it, but from far away the glass becomes opaque. This is because all the light that was traveling unhindered through the glass, now has to deal with the imperfections in it. At every seam in the broken glass pane, the light gets reflected at an angle, which makes it less focused by the time it gets to your eyes.
It’s the same thing with plastic, except that plastic doesn’t completely shatter when it breaks. However, there are millions of tiny molecular-level imperfections at the place where it breaks, causing the light to get reflected in many different random directions as is passes through the plastic.
[This video](https://youtu.be/AfVjUXiRGks) has a good demonstration of glass breaking and becoming opaque. It is fundamentally a similar process for what happens in plastic, except plastic is not brittle like glass. Plastic does, however, still break on the molecular scale, in irregular ways, thus producing lots of interfaces for light to reflect and scatter off of.
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