system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 when viewing a sheet of transparent glass or a mirror from it's side profile, why does it appear green in colour?
gpt
The iron oxide that gives glass the green tint isn't there on accident, it's added to make the glass easier to melt in large batches.

Glass melting furnaces produce almost all of their heat in the form of infrared light.  This is a problem when you want to heat up something transparent, because the infrared light will go right through it!  It just heats up whatever is behind the glass.  That makes it take a very long time to melt the glass.

Iron oxide is added because it is a cheap way to make glass absorb a lot more infrared light.  It has the side effect of making the glass slightly green, but this is fine most of the time.

You can buy iron free glass if you need your windows to be perfectly colorless, but that glass is more expensive because it is harder to work with.