Speed of light is an example of a light medium wave.
Most famous Michelson-Morley (1887) demonstrated that light travels at the same speed regardless of whether it was measured in the direction of the Earth's motion or at right angles to it. When the light travels through a vacuum: It moves from medium to medium (example: air to glass) speed can change depending of the new medium's index of refraction and the "bending" of light is essentially how lenses work.