The speed of light in a vacuum is presently defined to be exactly 299,792,458 m/s (about 186,282.397 miles per second). This definition of the speed of light means that the metre is now defined in terms of the speed of light. Light always travels at a constant speed, even between particles of a substance through which it is shining. Photons excite the adjoining particles that in turn transfer the energy to the neighbour. This may appear to slow the beam down though its trajectory in realtime. The time lost between entry and exit accounts to the displacement of energy through the substance between each particle that is excited.