system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: I remember my secondary school science teacher telling us that light can slow down moving through objects (prisms, water etc) if this is the case how is it a universal constant?
gpt
When we say "the speed of light," we usually mean the speed of light *in vacuum*. That is a universal constant. Light can pass more slowly through materials, but even then the speed will be constant for that material if the material is uniform.