table, ad/2, turned out to have a group of rays that all traveled
nearly the same distance.
As discussed in subsection 12.1.5, the principle of least time
is really a principle of leastor greatest time. This makes perfect
sense, since dL/dθ= 0 can in general describe either a minimum or
a maximum
The principle of least time is very general. It does not apply just
to refraction and reflection — it can even be used to prove that light
rays travel in a straight line through empty space, without taking
detours! This general approach to wave motion was used by Richard
Feynman, one of the pioneers who in the 1950’s reconciled quantum
mechanics with relativity. A very readable explanation is given in
a book Feynman wrote for laypeople, QED: The Strange Theory of
Light and Matter.826 Chapter 12 Optics