in this physics lecture, on slide 15-16, it is found that the ideal surface for a perfect lens (which maps a plane wavefront into a perfect spherical wavefront, i.e. which makes focus all input parallel rays into one point) can be an hyperbola or an ellipsoid according to the refraction index ratio being higher or lower than 1:
Now, I don't understand this result quite well. My doubts are:
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Imagine the rays start from right (being parallel). In case of an hyperbola, they are already in glass and then go into air. In case of an ellipsoid, they are in air and then go into glass. None of them are actually the common "thin lens" we usually study in basic optics (air – lens – air). How could we adapt these results to a thin lens? Should it have a hyperbolic/ellipsoid shape on both sides?
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I cannot visualize why a spherical surface is not ideal to map a plane wavefront into a spherical wavefront. A spherical surface slow down the input plane wavefront points in a spherical wave. I find strange that this does not occur, whilst the ideal surface are hyperbola and ellipsoid!
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Some books propose a different ideal surface for the perfect lens, precisely a cartesian oval. Other sources say the ideal surface should be parabolic, like for a mirror… which is the truth?
Best Answer
Spherical surfaces are used for lenses because they are much easier to manufacture precisely. The curvature is the same everywhere. You rub a spherical grinding surface all over the glass.
Lenses do have to be precise. An error of a wavelength of light matters. For visible light, that is about half a micron.
Fortunately, lenses typically use relatively large diameter spheres. In this case, a sphere is extremely close to the ideal shape.
For a small diameter lens, the approximation is close enough to perfect. For a larger diameter, the difference in shape is larger. The outer edge is a good match to an ideal shape with a different wavelength. The outer rays come to a focus in a different spot than the center rays. This is called spherical aberration, the aberration or error cause by using a spherical surface instead of an ideal surface.
To make an ideal lens from your left diagram, put a planar surface on the right side of the lens. Planar wavefronts will pass through a planar surface without changing direction.
For the right diagram, put a spherical surface centered on the focal point on the left surface. Spherical wavefronts will not change direction passing through it.
Of course this only works for waves parallel to the axis. There are other aberrations to deal with for off axis rays. And variations in wavelength also cause aberrations because the index of refraction varies with wavelength. Lens design gets complicated quickly. But for an application like focusing a laser, these simple lenses solve the problem.