Lenses

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These small drops of water focussed the almost parallel light from the sun.  They also provided a magnified image of the parts of the feather.  The smaller drops are almost spherical.  With an ideal lens the light from a point on an object would take the same time to reach the image point, whichever part of the lens it used.  But spherical surfaces cannot produce a perfect focus, which is a great pity, as they are the easiest curved surfaces to make. This behaviour is called spherical aberration.   Furthermore, because the refraction of light depends slightly its frequency, the colours are slightly dispersed, making a perfect focus even harder to achieve.  This is called chromatic aberration.

Several other problems occur in real optical systems, especially when wide angles or wide apertures are needed.  The traditional solution has been to use numerous pieces of glass, all with spherical surfaces.  By choosing glasses with different combinations of refractive index and dispersion, many types of lenses may be computed, using the radii of curvature and the thicknesses as degrees of freedom.  The problem of multiple reflections from the many surfaces is controlled using multiple coatings.  Nevertheless, especially with wide angle lenses, great care is needed to keep flare under control.

Recently it has become possible to make accurate aspherical surfaces at reasonable prices, greatly extending the range of options open to lens designers.

See also Surface Tension.