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How the
lenses came about ...
Lens (optics), in optical systems, glass or other transparent substance so
shaped that it will refract the light from any object and form a real or virtual image of
the object. Contact lenses and lenses in eyeglasses correct visual defects. Lenses are also used in the camera, microscope, telescope, and other optical
instruments. Other imaging systems may be effectively used as lenses in other regions of
the electromagnetic spectrum, such as the magnetic lenses in electron microscopes.
Lens Manufacture
Most lenses are made from special
types of high-quality glass, known as optical glass, that are free from internal strains,
bubbles, and other imperfections. The process of making a lens from a block of optical
glass involves several operations. The first step is to saw a lens blank from the glass
block. To form the rounded surface of the lens, the glass is then ground on concave or
convex iron tools charged with abrasive. A concave tool and a concave surface form a
convex lens surface by a convex tool. Commonly, two or more tools are employed in this
grinding process, using successively finer grades of abrasive. The final process of
finishing the lens surface is polishing, which is accomplished on a pitch-covered iron
tool coated with jeweler's rouge and water. After polishing, the lens is "edged"
by grinding the edge until the physical center and the optical center of the lens
coincide.
Lens Measurement
The optical characteristics of simple, or single, lenses or
compound lenses (lens systems containing two or more individual elements) are determined
by two factors: the focal length of the lens and the ratio of the focal length to the
diameter. The focal length of the lens is the distance from the center of the lens to the
image it forms of an object at an infinite distance in front of the lens. Focal length is
measured in two ways: ordinary units of length, as for example 20 in. or 1 m; or units
called diopters, equal to the reciprocal of the focal distance measured in meters. A
1-diopter lens has a focal length of 1-m (3.28-ft); a 2-diopter lens has a focal length of
0.5-m (1.64-ft). The ratio of the focal length to the diameter of a lens determines its
light-gathering power or "speed." This ratio is the so-called f-number of the
lens.
History
The earliest lenses, which were known to the Greeks and Romans,
were glass spheres filled with water. These water-filled lenses were used as burning
glasses. The processes used in lens manufacture have not changed essentially since the
Middle Ages, except for the utilization of pitch as a polishing medium, introduced by the
English mathematician Sir Isaac Newton.
Plastic lenses are cheaper, lighter, and less fragile than the
glass ones.
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