Wearing a pair of spectacles is a common sight. All men, women and children, who have weak eye-sight, use spectacles. Scientists have developed spectacles that help in seeing both the near and distant objects clearly. Nowadays, lenses are used inside the eyes in place of spectacles. These are known as contact lenses. These lenses can be of different colours and are used by those who do not want to wear spectacles.
The use of spectacles was started some 700 years ago. In 1266, Roger Bacon of England used a piece of glass to magnify the words written in a book. This glass piece was cut out of a spherical ball of glass. But it is not definitely known as to when glass pieces were used in the form of spectacles.
Spectacles are shown on the eyes of one Cardinal Ugon’s portrait made in 1352. This proves that spectacles were developed during the period 1266-1352. By the sixteenth century, they were very much in use. In 1784, Benjamin Franklin brought wonders in the field of spectacles by making bifocal lenses. A question must be coming to your mind as to how do we see clearly with the help of a pair of spectacles?
It is so simple. Our eyes act as a camera. The light rays enter our eyes through cornea (black portion of the eye). There is a convex lens inside the eye, and behind this lens there is a screen which is called the retina. The light rays coming from any object make an inverted image of the object on the retina with the help of this lens. This image is carried to the brain by the optic nerve. It becomes erected there. This is how we see an object. If the eyes have no defect, the focal length of this lens gets automatically adjusted and the image of the object always falls exactly on the retina. But, sometimes, the eyes develop some defects due to which the image of the object is formed before or behind the retina. Thus, the object appears blurred. Persons having such eye-defects need the assistance of spectacles. The defects of the eyes are mainly of three types.
Myopia: Persons suffering from this defect can see the nearby objects clearly, but the distant objects appear blurred. The image of the objects due to this defect is formed before the retina. This defect is corrected by using concave lenses in the spectacles so that the image of a distant object is formed on the retina.
Hypermetropia: Persons suffering from this defect cannot see nearby objects clearly but can see distant objects clearly. Short distance objects make their images behind the retina. This defect is corrected by using convex lenses in the spectacles so that the image of the object is again formed on the retina.
Astigmatism: Due to this defect in the eyes, one cannot see with clarity the horizontal and vertical lines simultaneously. Spectacles with cylindrical lenses are used to correct this defect.
Bifocal lenses have both concave and convex lenses so that these can be used by people who are suffering from both myopia and hypermetropia.
In this way, spectacles can help man to see objects clearly. Spectacles have proved to be a great boon for persons with weak vision. In addition, to protect the eyes from the intense rays of the sun, goggles, cooling glasses (sun glasses) are used. Coloured glasses are used in these, which prevent the ultra-violet rays of the sun from entering the eyes. The ultra-violet rays are very injurious to the eyes because they can burn the tissues due to their high energy content.