On Mon, 15 Oct 2012 16:12:08 -0400, Joseph Gwinn
wrote:

In article ,
wrote:

I was watching a lecture about QED given by Richard Feynman. Part of
the lecture was about the reflection of light. This is what I think he
said, paraphrased: When shining a light at a reflective surface the
probabability that the light will take the shortest path or the
longest path to a detector, placed anywhere, is equal. However, the
AMPLITUDE of the probability is not equal, but varies. The light
particles with the highest amplitude are the ones we see when the
incident angle and the exit angle are the same, the shortest path. So
I think Feynman said the probablity can be the same but the amplitude
of the probability can be different. Is that correct? If so, can
anyone point me to a web site that explains this in a way a layman can
sort of understand?

Feynman was stating Fermat's principle, but in a quantum mechanics
framework.

http://en.wikipedia.org/wiki/Fermat%27s_principle

What Fermat actually said was that the actual path is an extremum, which
can be the longest or the shortest. In practice, it's always the
shortest. An extremum is a point where the slope of path length versus
choice of nearby path is zero.

If you want a bigger explanation, try posting the question on sci.optics.

Joe Gwinn
What I'm asking about Joe is the statement: The probabilities are
equal but the amplitude of the probabilities are not. Is this correct?
I understand that the light can be reflected along longer paths, that
diffraction grating can be used to show this. It's this amplitude
thing that I'm not sure about.
Eric