>"In a quantum-mechanical picture, light consists of photons, or packages of optical energy. The photons of the light reflected from a metal (or a dielectric mirror) are identical to the incident ones, apart from the changed propagation direction."
This doesn't explain anything about how it works quantum mechanically.
They are identical in terms of their quantum mechanical properties.
you are stating somehow the direction of photon is preserved when absorbed by electron - absurd idea even for layman physics (not claiming I know how this works, but this can't be the way)
edit: to all the downvoters of saiya-jin — let the one who has never defended their incorrect intuitions in physics cast the first downvote!
Amen. This stuff is wildly counter-intuitive, we only properly know how mirrors work since we understand the photo electric effect, and even with that understanding it is still quite tricky because it requires insight into how stuff works at a level where direct observation is no longer possible without access to enormous resources.
Of course you can now argue that if the photons hit the metal they will not pass through, but that's not how it works: the photons will excite an electron to a higher orbit and it may drop back to a lower orbit on the other side of the film making the metal appear transparent or it may reflect.
edit: saiya-jin I can't reply to your comment but yes, the direction is preserved. The same happens with a mirror, the photons ejected will be ejected at the correct angle even though the photoelectric effect has absorbed the photons. That's why metals reflect the way they do!
https://www.scientificamerican.com/article/how-do-mirrors-re...
The small fraction of photons that is absorbed will heat up the mirror.