“There was a young lady named Bright,
Whose speed was far faster than light;
She set out one day
In a relative way,
And returned on the previous night.” -A.H. Reginald Butler
Throughout the entire Universe, there’s a fundamental law that governs the motions of all particles: Einstein’s relativity. It states that all particles with mass can never attain the speed of light, no matter how much energy you put into it. Additionally, all massless particles only move at the speed of light, no matter what you do to either them or to the device/person observing them. No matter what reference frame you’re in, the speed of light in a vacuum is a constant.
But light isn’t always in a vacuum! From air to quartz to acrylic to glass to many other media, light can pass through transparent material, and when it does, it slows down. Not only that, but light of different energy slows down by different amounts. In what ways is the speed of light always the same, and in what ways can it change?