When reviewing a refractive index chart there is a relationship to the density of the medium and the velocity of electromagnetic radiation through the medium. The lower the density the greater the velocity. The less stuff in light's path the faster it goes. The vacuum medium, the QED electromagnetic field sometimes called the photon field, propagates light at 300 million m/s.
My question in very layman's terms is this: Stuff in light's path slows it down, but if there is no stuff then there is no field and propagation stops. How does modern physics look at this transition zone from a very low density field to zero density. Would a less dense photon field increase the speed of propagation? Does the photon field have a refractive index?