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Waves on a spherical refracting surface. Suppose that light is incident parallel to the optic axis. The ray coming in along the optic axis is not deflected. The ray incident at a height y above the optic axis bends as shown. With air on the left side and glass on the right we have.
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The ray coming in along the optic axis is not deflected. The ray incident at a height y above the optic axis bends as shown.
This is independent of y, so all incident rays that hit the spherical surface parallel to the optic axis are refracted through F
All the light is focused at one point, lying on a plane perpendicular to the optic axis, the so-called focal plane.
Recall that so that From the geometry,
n1 When the external medium has a refractive index n1, the corresponding formula is n2