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It depends on the material used in its manufacture. In the case of glass or crystal models, thick slices of these materials are broken by a special saw. The pieces are cut into circular shapes and heated until pliable. Afterwards, they are placed in a mold of the size and curvature that you want to give the lenses. Then, the surfaces are sanded with a chemical compound made up of carbon and silicon, or with a rough emery, to acquire the final shape. Fine polishing is done with a special abrasive flour. Lastly, the edges are trimmed to center the lenses and give them the correct diameter, so that the light beams that pass through them are focused on a single point – the retina. Variations in curvature and thickness determine lens power. The basic manufacturing process for acrylic lenses, another fairly common material, is not much different.
The resin is heated and placed inside molds in the shape of the lens. After being cooled, the acrylic is also polished with special sandpaper, so that the lens acquires the ideal curvature.