I think that's a common misconception because light actually does travel slower in different mediums such as water and glass and it's not because it's taking a longer path. Light is an electromagnetic wave (EM wave) and when these wave come in contact with an object (like glass), the waves cause the electrons of that object to oscillate. Since electrons have charge, they also have an electric field associated with them. But now that light is causing electrons to move about, the electric field associated with the electrons start to vary and because of that varying electric field, the electrons produce their own electromagnetic wave (light)...but these new EM waves from the atoms are out of phase with the EM waves of light (ex: one EM wave looks like cos(theta) while another EM wave which is out of phase looks like cos(theta + pi/2)). If you were to add all those EM waves together, then the net result (the superposition of the waves) would be an EM wave that has a slower phase speed than that of an EM wave in space. [spoiler]If what you said was true, then the amount of time it takes light to travel through an object would be completely random because the amount of time an atom takes to absorb and emit photons is almost random. So, sometimes while we're shining a light through an object, we would see the flow of light just stop for a very short time or get more intense...which is not the case in reality. [/spoiler]