What is the difference between optically flat surface and atomically flat surface?
Atomically flat surfaces would suggest that the surface is flat on an atomic level which is a nearly if not completely impossible accomplishment. Optically flat suggests that the surface is flat to the human eye such as glass or ice. Note that even though glass and ice looks flat and smooth to the naked eye, they are in fact covered with numerous molecular impurities and are thus not atomically flat.
No. Here is one definition: http://en.wikipedia.org/wiki/Optical_flat
"Optical flats are optical-grade glass structures lapped and polished to be extremely flat on one or both sides, usually within a few millionths of an inch..."
Commercial optics have surfaces specified to be flat to within some fraction of a wavelength. Typically this is between 1/4 to 1/20 of some specified (usually visible) wavelength. As such, the flatness is on the scale of 30 to 140 nm.
Since it is impossible to be flatter than the size of an atom, I presume "atomic flatness" means flat on the scale of an atom, or roughly 0.1 nm.
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