Most likely, they want you to calculate the difference in vergence from the two lens surfaces, as seen through either side of the lens. When observing the depth of an object within a medium, which in this case represents the depth of the opposite surface of the lens as viewed through the lens material, the apparent depth or
reduced thickness can be calculated using the conjugate foci formula:
where
d represents the reduced thickness,
n represents the refractive index of the lens material,
r represents the radius of curvature of the surface toward the viewer in meters, and
t is the real center thickness of the lens in meters. This assumes that the surrounding medium (air) has a refractive index of 1.
If you solve this equation for
d using both lens surfaces, you can calculate the difference in apparent depth or reduced thickness when viewing the lens through either surface.
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