Consider a photon emitted by a photon emitter, it travels through a vacuum and then into a block of glass, it travels slower through this than the vacuum right? It leaves the block of glass and continues through the vacuum until it hits its target. Q1. When it leaves the block of light where does it get the energy to accelerate?  Q2.  Where does it accelerate? If it does not how can it go from one speed to another without doing so?  Q3. What is it's speed when it is halfway out of