system
Given the following post, respond as if you're commenting on Reddit:
human
Why are the cracks on glass opaque?

If the air is see-through, and the glass is too, why are cracks not?
gpt
Light moving between different mediums (air, water, glass) gets a little bit bent; this is why drinking straws look bent when put into a glass. When the glass pane is smooth, the bend is minimal, since it only transitions twice (in and out). When it breaks, the glass becomes super rough, and air and glass get jumbled up. Now light is making that transition hundreds of times, so it gets all bent out of shape. All of the colors sort of blend together, giving a ‘white’ appearance. This is also why snow is white.

Edit: Polar bears have black skin and transparent hair.