When were eyeglass and reading glass lenses developed?

The first high-quality optical glass was developed in Germany toward the end of the 19th century. Prior to that time, eyeglass lenses were made from Brazilian quartz crystal and were poor in quality. Until 19l4, most glass used in optical lenses was imported from Germany. At that time, John Bausch and Henry Lomb developed a high-quality glass in their Rochester, N.Y., factory.

Glass remained the primary material for eyewear lenses until the 1940s when the Pittsburgh Plate Glass Company developed a plastic lens material named CR-39. This material remains the choice for most lenses today because it weighs about half as much as glass and is more impact-resistant. Like glass, it can also be ground, polished, and tinted.

The other plastic used in lenses is polycarbonate, a lightweight, almost unbreakable material developed by NASA as part of the space program. This is the material commonly used in children's, sports, and safety glasses. A major advantage of poly is that it can be ground much thinner than either glass or CR-39. It is, however, more expensive and usually needs an anti-scratch coating because it is softer.

How does an reading glass lens work?

The focusing ability of an eyeglass lens is determined by the difference in curvature on its front and back surface, its thickness, and its index of refraction. This index varies according to the density of the material from which the lens is made. The degree to which a given lens material bends light is its index of refraction (IOR). The higher the IOR, the greater the light-bending ability, and usually, the greater the cost of a lens.

An eyeglass lens surface is either spherical or cylindrica.  A spherical surface is similar to the shape of a ball in that it has the same degree of roundness throughout its surface. It can be either convex, which means it curves outward, or concave, which means it curves inward. A concave surface is always on the back side of a lens. A cylindrical surface is not evenly curved, but more like the shape of an egg or a football. Spherical lens surfaces are sometimes called toric surfaces.

Convex lenses are used to correct hyperopia or farsightedness, while concave lenses help correct myopia or nearsightedness, and cylindrical lenses correct astigmatism.

4readers.com