Physicists and mathematicians discovered centuries ago that spherical sections of glass could bend and focus images onto a flat field. That discovery enabled lens designs well into the Twentieth Century. For instance, the Sonnar-type lens for my Nikon S from the 1950s has seven elements, all circular in cross section.
The combinations of lens elements provide a high degree of resolution along with minimizing aberrations with the widest possible aperture for each design type. Glass of great purity was essential to the process of sharp image formation.
The lenses in my Iphone also have seven elements, but they embody complex aspherical shapes made possible by computer aided design and ultra-miniaturazation. It would be physically impossible to form such tiny lenses using the mechanical polishing processes for glass lenses.
Instead, the cellphone lenses are made of injection molded plastic with very high light transmission qualities.
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