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Optical Lenses

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A lens is a transparent refracting medium, bounded by one or two surfaces, one or both of which are usually curved. Early lenses were made of glass, but more recently, plastic materials have begun to be used. Such a device allows a beam of light rays to converge or diverge on passing through it.

Why the name lens? Some of the earliest lens makers were Italian. It is claimed that because Italian lenses were bi-convex, they resembled the lentils they used to make soup - so 'lens' came from the Latin for lentil (Lens culinaris).

Traditionally, the history of advances in lenses has focused on 16th- to 18th-Century Europe, particularly in Italy and the Netherlands, but there have been some anomalous archaeological discoveries that contradict this simplistic view1.

From Burning Glasses to Microscopes

Lenses were no doubt being made soon after the art of glass-making was first discovered, which was prior to 2000 BC.

The earliest lenses, which were magnifying lenses of a sort, were known to the Greeks and Romans. These glass bowls filled with water were used as burning glasses2; but their effect of enlarging details was well recognised3. The great comic dramatist, Aristophanes, mentions a similar burning glass:

STREPSIADES: Have you ever seen a beautiful, transparent stone at the druggists', with which you may kindle fire?
SOCRATES: You mean a crystal lens.
STREPSIADES: That's right. Well, now if I placed myself with this stone in the sun and a long way off from the clerk, while he was writing out the conviction, I could make all the wax, upon which the words were written, melt.
- Aristophanes in the play Comedy of the Clouds, Act II (424 BC)

Furthermore, Pliny relates that such bowls were used by physicians for burning. The glass bowl was obviously used as a condensing lens, though it was no doubt a marvel to the Romans that cold water should be able to cause a burn! So, however dimly, the biconvex lens was already known in classical times.

Ancient Lenses

In 1998, archaeologists discovered clear quartz discs at a Viking settlement in Sweden, dating from 700 - 1000 AD. These were at first thought to be jewellery, but optics specialists led by Olaf Schmidt of the University of Applied Science in Aalen, Germany, have discovered that they are, in fact, sophisticated lenses. Their shapes closely match that of an ellipse. The optical properties of some of them are comparable to modern lenses. Therefore, Vikings were making lenses hundreds of years earlier than first thought. Schmidt believes the lenses were made on a simple lathe and used for focusing sunlight to cauterise wounds and light fires.

Two lenses of optical quality are on display at the Heraklion Museum of Ancient Cretan Civilisation. As many as fifty were reported as having been found by Heinrich Schliemann in the excavations of Troy, though only a handful have been properly published.
- Peter James and Nick Thorpe, Ancient Inventions (Ballantine Books, 1994)

Some lenses from these sites have impressive magnifying powers. One lens, probably of the 5th Century BC, found in Crete, can magnify with perfect clarity up to seven times. If it is held further away from the object viewed, it will actually magnify up to twenty times, though with considerable distortion.

The processes used in lens manufacture have not changed essentially since the Middle Ages, except for the utilisation of pitch as a polishing medium, introduced by Sir Isaac Newton.

The First True Lenses

However, true glass lenses were not known in classical times; they were probably first manufactured at the end of the 13th Century in Europe, and Roger Bacon's name is associated with this. Bacon discussed the use of segments of spheres and showed that letters and small objects on which they are placed appear to be magnified.

For this reason such an instrument is useful to old persons and to those with weak sight, for they can see any letter, however small, if magnified enough.
- Roger Bacon, Franciscan friar and scientist

This observation of the magnifying properties of segments of spheres was not original to Bacon. His real contribution was the clear recognition of their use for old people and those with weak sight. Although it has been claimed that Bacon actually invented spectacles this seems to be a myth. For more information on this, click on the link.

The lens surface of spectacles can be either spherical or cylindrical. A spherical surface has a uniform curvature across its surface. The equations governing the optical properties of lenses are thought to have been devised by the French philosopher and mathematician René Descartes in the 17th Century. But he could not find anyone skilful enough to make one.

Improvements on the Basic Model

In the 17th Century, the best manufacturers of lenses in the world were in the Netherlands. Foremost among these was Anton van Leeuwenhoek (1632 - 1723) who made his own simple microscopes using only one lens. He became an expert at making tiny lenses with strong magnifying power. Objects were mounted on the point of a pin and brought into focus of the lens by adjusting screws to the pin. Leeuwenhoek later added a second lens to produce simple 'compound microscopes' and as a result became known as the 'father of microscopy'. He was the first person able to see and describe cells and microorganisms.

Later on, in Britain, Robert Hooke (1605 - 1703) improved Leeuwenhoek's design by adding a third lens behind the original two. Optical microscopes today usually use more than one lens, in series.

The most common types of glasses used in optics are crown glasses - which are composites of silicon dioxide (silica), sodium oxide (soda) and calcium oxide (lime) - and flint glasses. Prior to the mid-18th Century only crown glass was available. However, by 1758, flint glass, which contains lead oxide and which is denser and disperses light into a spectrum more strongly, started to become available. This enabled the spectacle-maker, John Dolland (1706 - 1761) to manufacture small 'achromatic' telescopes. Although flint glass had long been in use for making bottles, it had been difficult to manufacture in a quality suitable for lenses.

It was only the efforts of a Swiss glass-maker, Pierre Guinand (1748 - 1824) that allowed a significant improvement to occur. Guinand devised a process for stirring the glass while it was molten, and so removed the bubbles and unevenness in the mixture. This was so successful that he applied his method to the manufacture of all optical glass, and during the 19th Century he set up a works at Benediktbeuren, in Bavaria, together with a German manufacturer whose manager was Joseph von Fraunhofer (1787 - 1826). Here, excellent optical glass was made and Fraunhofer produced both optical lenses and also the facility for mounting them. His telescopes were of such high quality that both Friedrich Bessell (1784 - 1846) and Friedrich Struve (1793 - 1864) used them for measuring stellar distances.

Recent Developments

The recent development of plastics and of special processes for moulding them has led to their increasing use for the manufacture of lenses, for example, contact lenses. Plastic lenses are cheaper, lighter, and less fragile than glass ones. However, they suffer from the aesthetic disadvantage that they are thicker than glass lenses of equivalent power.

1The Ages of Science. Colin A Ronan. Harrap, London (1966).2A burning glass is a large convex lens which can focus the sun's rays on a small area and so ignite materials.3An early reference can be seen in the works of the Roman philosopher, Lucius Annaeus Senecus, 3 BC to 65 AD.

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