Several experiments are carried out to discover the cause of convexed deformation of lapped glass plates.
A “Permanently Strained Layer” is formed on a glass surface by lapping.
The layer is expansive and tends to increase its area causing convexed deformation of the plate. The layer has compressive stress because of the rest part of the plate resisting the expansion of the layer.
The layer has greater refractive index and lower chemical stability than those of the glass substance.
These lead to the following theoretical equation. _??_
where _??_: Initial curvature of the glass plate.
_??_: Curvature after lapping.
T: Thickness of the glass plate.
S: Sum of permanent strain for the thickness of the permanently strained layer.
c: Depth of the center of
S from the surface of the plate.
Curvature variations of thin glass plates which were lapped under various conditions are also studied with the result that:
The curvature variation of any glass plate, in whatever condition it might be lapped, undergoes a similar progress explainable by the above equation.
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