Hmm. Interesting. Gone through the maths, and taking into account the doppler shift of the photon seems to satisfy both energy and momentum conservation, at least in the initial situation when the mirror starts to move.
Hmm - cool. Well, I guess maybe it was an easy question then :) I...
I am currently doing a PhD on the subject of solar sailing (if you don't know what that is and are curious enough, a quick google search on "Solar sailing" will give you a plethora of websites that will explain it for you). Solar sails depend crucially on forces generated when a photon is...