system
Given the following post, respond as if you're commenting on Reddit:
human
Why do people say that when light passes through another object, like glass or water, it slows down and continues at a different angle, but scientists say light always moves at a constant speed no matter what?
gpt
We're just speaking loosely.

The speed of light *in a vacuum* is a universal constant. It is also "invariant", which means it doesn't change depending on your reference frame - i.e. it doesn't depend on your speed or location.

The actual speed of light through a medium - not just the abstract theoretical limit of "speed of light in a vacuum" - can change depending on the medium, and isn't a universal constant.

Edit: To clarify further, it might seem a bit odd that so much of physics depends on light, which is after all just one type of specific phenomenon. But really that's backwards. "c" is a special universal constant that tells us about the relationship between space and time, the propagation rate of information and so on. It just so happens that some phenomena - such as electromagnetic waves - will travel at *c*, under idealised circumstances. That is, relativity isn't really about light itself, it's just that light is strongly affected by relativity so it provides a useful way to work out what relativity does.