🚀 go-pugleaf

    Someone explained to me once why the photo-electric effect doesn't
    definitively rule out a classical theory for E&M radiation, but 
    I've forgotten. Can anyone briefly explain why again? Thanks.

Depends what you mean by classical.  If you assume that atomic energy
levels are quantized, and make various plausible assumptions about the
interaction between the atoms and the EM field, you can derive the
photo-electric effect, while still assuming that the EM field in free
space is fully classical.  In fact Planck did just that, in his
so-called "second theory" (which he put forth *after* Einstein's
photon hypothesis).

The rise and fall of Planck's second theory is described in Kuhn's
book _Black Body Theory and the Quantum Discontinuity_.

If I recall correctly, Planck's second theory was where Zero Point
Energy made its debut.  (Kuhn discusses this.)  I've also heard it
said that Planck's second theory was the forerunner of Stochastic
Electrodynamics (SED).

The Compton effect was historically the nail in the coffin of such
semi-classical explanations.  But I disagree with Martin Green when he
writes:

    There is NO honest way of helping high school students understand
    the dual nature of light, because all the "ordinary" quantum
    effects can be explained by the quantum properties of matter.

I think Martin Green is confusing "evidence for" with "mathematical
proof of".  I believe it is intellectually honest to tell high-school
students that Planck's black-body formula, and the photo-electric
effect, and a bunch of other stuff, lead to our current picture, which
explains vast reams of phenomena remarkably well.  That is
historically accurate, and in keeping with the nature of scientific
proof.  (I leave scare-quotes off `proof' deliberately---
mathematicians don't own the copyright on this word.)