&quot;To keep track of time along light paths, I will be using a slightly modified geometric optics model, in which light travels along straight lines at constant speed.&quot;<p>This is computer-rendering light, not real light.<p>A careful watching of the first video can incidentally reveal why we don&#x27;t simulate real light. Watch as the first clean pulse of light spreads out and goes from this really-easily-characterizable expanding circle to an ever-more-complicated set of waves as it moves out into the world. You can&#x27;t afford to simulate things as waves, at least, not with conventional computers.