Commons

in their passing out of one medium into another.” But a rayis a record of the path traveled by light, not a physical thingthat can be bent or broken.• In theory, rays may continue infinitely far into the past andfuture, but we need to draw lines of finite length. In j/1, ajudicious choice has been made as to where to begin and endthe rays. There is no point in continuing the rays any fartherthan shown, because nothing new and exciting is going tohappen to them. There is also no good reason to start themearlier, before being reflected by the fish, because the directionof the diffusely reflected rays is random anyway, and unrelatedto the direction of the original, incoming ray.• When representing diffuse reflection in a ray diagram, manystudents have a mental block against drawing many rays fanningout from the same point. Often, as in example j/2, theproblem is the misconception that light can only be reflectedin one direction from one point.• Another difficulty associated with diffuse reflection, examplej/3, is the tendency to think that in addition to drawing manyrays coming out of one point, we should also be drawing manyrays coming from many points. In j/1, drawing many rayscoming out of one point gives useful information, telling us,for instance, that the fish can be seen from any angle. Drawingmany sets of rays, as in j/3, does not give us any more usefulinformation, and just clutters up the picture in this example.The only reason to draw sets of rays fanning out from morethan one point would be if different things were happening tothe different sets.j / 1. Correct. 2. Incorrect: impliesthat diffuse reflection onlygives one ray from each reflectingpoint. 3. Correct, but unnecessarilycomplicatedSection 7.1 Light Rays 141