The speed of light actually depends on what material the light is moving through! Each material has a property called an index of refraction. From the index of refraction, we can calculate the speed of light passing through a material from the following equation:

In this equation, c is the speed of light in a vacuum (about 2.998 × 10⁸ m/s), n is the index of refraction of the material in question, and ν tells us the velocity of light in the material.