Re: Surface Roughness & CP's

Most of the interferometer tests that I have seen of flats show an extremely flat surface. The biggest contributer to the P/V is usually the edge, where a slightly turned edge is quite common. But the profile of the main surface is very smooth with little ripple or ridges. With RMS of 1/100 or better (I have one with a 1/333 RMS), I doubt that there would be much of an effect to the image.
I was once told that when dealing with differences of less than 1/20 wave that very little is introduced by the flat.
I am not an expert at this of course, just quoting one.

Jean Texereau in a 1950 paper states:

"Optical surface faults of small amplitude (micro-ripples) can have a height of the order of 1/1000 of a wavelength and a half-width of the order of a millimeter, with a slope of 10 exp -6 to 10 exp -5.
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Herbert:

A bit of basic math involving Avogadro's number (6.02 x 10^23 molecules per gram molecular weight), the density of silicon dioxide (~2.2 gm/cc) and the molecular weight of SiO2 (60) indicates to me the molecular spacing would be on the order of about 0.356 nanometers. The surface roughness you quote would seem to be on the order of 0.55 nanometers.... Something to consider.

I think at these dimensions one can no longer look at these materials as homegeonous materials. I think it is necessary to change modes of thinking here and consider what polishing/smoothness means on the molecular level, how light interacts with materials on an atomic/molecular level. The approximations and assumptions that work when one is looking at 1/10 wave optics or even 1/50the (dreaming IMHO) are probably no longer valid when operating near the molecular spacings.

Personally I have a difficult time believing that a 1/1000th wave error of any sort has much meaning.

jon isaacs

When you look through an electron microscope, everything is rough surfaced. But at these small deviations from flat, how much does it effect light waves as they reflect from this surface? How much would effect the actual view through the eyepiece? I think it would not be visual myself and I think that even worrying about this type of roughness is a waste of energy and time.

Hello Herb,

I'm very interested in these comparisons. I've seen photos of bad mirrors to illustrate the technique but never phase contrast photos of what passes for saleable glass. Could you post a photo of the micro-roughness of the photographic filters?

Cheers, Eric