In a Nutshell
Most believe that the speed of light is constant and unchanging, but it simply isn’t the case. The speed depends entirely on what exactly the light is traveling through.
The Whole Bushel
It is often quoted that the speed of light is constant, and the misconception is understandable. The speed of light IS constant, but only in a vacuum. The speed of light in a vacuum is 299,792,458 meters per second and this number remaining constant is important in because it is the speed used to correctly define the length of the meter.
But what happens to the speed of light when not traveling through a vacuum? It changes, a lot. A vacuum contains no matter, but if the area the light is traveling through DOES contain matter such as dust or moisture, the light will bend and decrease in speed. When light travels through a diamond, it slows to less than half it’s vacuum speed.
But it can be slowed even further when passed through a type of matter most haven’t heard of called a Bose-Einstein condensate or ‘bec’. This ‘new’ matter was first proposed in 1924 by Albert Einstein and Satyendra Nath Bose. Light passing through a bec can be slowed to under fifty miles an hour. In 2000, a team at Harvard university managed to bring a beam of light to a complete standstill by passing it through a bec version of Rubidium.
So the speed of light isn’t constant and in the right conditions, you could probably even run faster.