Flatness (liquids)

Flatness refers to the shape of a liquid's free surface. On planet Earth, the flatness of a liquid is a function of the curvature of the Earth, and from trigonometry, can be found to deviate from true flatness by approximately 19.6 nanometers over an area of 1 square meter, a deviation which is dominated by the effects of surface tension. This calculation using the Earth's mean radius at sea level, however a liquid will be slightly flatter at the poles.

References

Dew, G.D. (March 1966), "The measurement of optical flatness", Journal of Scientific Instruments, 43 (7): 409–415, Bibcode:1966JScI...43..409D, doi:10.1088/0950-7671/43/7/301, PMID 5941575
Bünnagel, R.; Oehring, H.-A.; Steiner, K. (1968), "Fizeau interferometer for measuring the flatness of optical surfaces", Applied Optics, 7 (2): 331–335, Bibcode:1968ApOpt...7..331B, doi:10.1364/AO.7.000331, PMID 20062467

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