because light is a wave and a particle does that void the bottlenecking of it being in a medium
Light slows down depending on the medium.
That is why if you shine a beam of light through a perspex prism, it will split into a rainbow, as it is slowed down, and the different wavelengths of light seperate.
The speed of light is only the speed of light when in a vacuum.
In our atmosphere light travels very slightly slower.