Physics Asked by Faissaloo on December 10, 2020
I keep reading this in comparisons of different kinds of lenses but I don’t understand why the flat side would change the amount of spherical aberration if the focal length still has to be the same?
Plano-convex lenses only reduce the spherical aberration if they are used in the appropriate position with respect to the conjugates. To understand this, one intuitive explanation is this: Snell's law is not linear, so it does not behave proportionately for large versus small angles.
Looking at the two lenses below, for the left hand lens the angles of deviation for the marginal ray (the ray at the edge of the lens) are distributed between the front and back surfaces. For the right hand lens, the 2nd surface performs all of the work of refraction. It turns out the right hand lens will have much more spherical aberration than the left hand lens, when used in this manner.
This is a simplified example, but more complicated lenses are designed by adjusting the amount of refraction at each surface, ideally so that aberrations on one surface are compensated by aberrations on another surface.
Correct answer by JB2 on December 10, 2020
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