An ellipse of the dark area is always seen whenever the light is allowed to pass through bifocal glasses [Fig. 1a- white arrow]. This ellipse corresponds to the junction of the distant and near lenses and we believe it to cause some form of scotoma to the person whose mechanism [Fig. 1b] we hypothesize to be very similar to roving ring scotoma,[1] in case of aphakic glasses. Differential refraction, causing differential prismatic deflection, at the junction of 2 optically dissimilar surfaces forms a scotoma between the 2 zones as shown. The same mechanism partly also contributes to negative dysphotopsias.[2]
Figure 1.
(a) Light across bifocal glasses showing the scotomatous area (white arrow). (b) Ray diagram explaining the mechanism for the same
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References
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