Fermat's Principle in Optics Fermat's principle in optics
states that light always travels from one point to another along a
path that minimizes the travel time. Light from a source A is
reflected by a plane mirror to a receiver at point B, as shown in
the figure. Show that for the light to obey Fermat's principle, the
angle of incidence must equal the angle of reflection, both
measured from the line normal to the reflecting surface. (This
result can also be derived without calculus. There is a purely
geometric argument, which you may prefer.)