Light travels in straight line ..this property is called rectilinear propagation of light....it needs particles the air to travel...if there are no particles in the air ex.vaccum ..one cannot see light as it needs particles to scatter it..this is called tyndall effect.
The "medium" you are refering to is physical. Energy is not something physical. Albert Einstein gave us the RELATIONSHIP between the two, not a way to change from one to the next, "E= MC^ 2". The two are extremely seperate from each other. Some scientists say there is a "particle" for light, others say it's a "reaction". That's what I know.