Does light speed up or slow down when it travels from air to liquid?

It slows down

Explanation:

According to Einstein's theory of relativity. "Nothing can travel faster than the speed of light in a vacuum which is indicated by c." "Light in a vacuum always travels at the same speed." c is the ultimate speed limit of the universe.   As light travels through different medium such as water, diamond etc, it scatters off of the molecules in the medium and is slowed down. For some medium such as water, light will slow down hence a reduction in speed. The amount by which light slows in a given medium is described using the index of refraction, n. The higher the index of refraction, the slower the speed of light. Speed of light in vacuum is approximately 3.0 x 10⁸meters per second, with a refractive index of 1.0, but reduces to 2.3 x 10⁸meters per second in water (refractive index = 1.3) and 2.0 x 10⁸meters per second in glass (refractive index of 1.5). The Speed of light is lowest in diamonds.