Elucidation of Some Tricky Points in
Richard Feynman's book QED

This World Wide Web page written by Dan Styer, Oberlin College Physics Department;
last updated 21 June 2012.

Page 14, paragraph beginning "The human eye is a very good instrument . . .":
A discussion of light reception by the human eye is
F. Rieke and D.A. Baylor, "Single-photon detection by rod cells of the retina,"
Reviews of Modern Physics 70 (July 1998) 1027-1036.

Page 15, paragraph beginning "I want to emphasize that light comes in this form . . .":
I think that Feynman makes a pedagogical error here when he insists that light is a particle. True, one always gets correct results by considering light to be a particle: not as a familiar classical particle like a marble, but as a strange quantal particle that, for example, might not have a position. However, the word "particle" so strongly suggests the classical marble that this passage gives the wrong impression. I far prefer the way Feynman himself treats the same issue in "The Character of Physical Law" (MIT Press, 1965, page 128):

Now we know how the electrons and light behave. But what can I call it? If I say they behave like particles I give the wrong impression; also if I say they behave like waves. They behave in their own inimitable way, which technically could be called a quantum mechanical way. They behave in a way that is like nothing that you have ever seen before. Your experience with things that you have seen before is incomplete. The behavior of things on a very tiny scale is simply different. An atom does not behave like a weight hanging on a spring and oscillating. Nor does it behave like a miniature representation of the solar system with little planets going around in orbits. Nor does it appear to be somewhat like a cloud or fog of some sort surrounding the nucleus. It behaves like nothing you have ever seen before.

Page 38, paragraph beginning "We start with a mirror . . ." (see also figure 19 on page 39):
Feynman's light source is (potentially) sending out photons in all directions, like the filament in a light bulb and unlike a spotlight or a laser that sends light out in only one direction.

Page 87, paragraph beginning "As for the time scale . . .":
Feynman's time T is what we would normally call "ct", where c is the speed of light and t is the time measured in seconds.

Pages 87-91, amplitudes for the first two basic actions:
Here is a technical discussion. (Reading this discussion requires the free Adobe Acrobat Reader software.)

Pages 115-118, magnetic moment of the electron:
Here is an update and elaboration. (Reading this document requires the free Adobe Acrobat Reader software.)