We often come across people wearing spectacles. Wherever we go, it is possible to spot a person with spectacles within a moment. Anyone with weak eyesight uses spectacles. These days, there are spectacles available which help in viewing both near and far objects. Nowadays, lenses that fit externally over the cornea of the eyes are often used in place of spectacles. These are known as contact lenses. These lenses can be of different colors and are used by those who do not want to wear spectacles, especially the youngsters.
Lenses help the image to form on the retina
The use of spectacles started over 700 years ago. In 1266, roger bacon of England Italy
Cardinal Ugon’s portrait made in 1352 shows him wearing spectacles. This proves that spectacles were developed during the period 1266-1352. By the 16th century, they had become very common. On 1784, Benjamin Franklin brought about a wonderful development in the field of spectacles by making bifocal lenses- a lens with two halves for both near and distant vision. You must be wondering how spectacles help us to see clearly.
Our eyes act like a camera. The light rays enter our eyes through cornea. There is a convex lens inside the eye, and behind this les there is a light-sensitive screen, called the retina. The light rays coming from any object form 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. The brain converts these images as erect. This is how we are able to see an object correctly.
If the eyes have no defect, the focal length of this lens gets automatically adjusted and the image of the object always fails exactly on the retina. But sometimes, the eyes develop some defects formed in front or behind the retina. Thus, the object appears blurred. Persons with such defects have to use spectacles. There are mainly there defects of vision that can be corrected by the use of suitable eyeglasses or spectacles.
Myopia: persons suffering form this defect can see nearby objects clearly, but distant objects appear to be blurred. This defect causes the image of an object to form 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. Usually students suffer from this defect.
Hypermetropia: people suffering from this defect cannot see nearby objects clearly, but distant objects are clearly visible. Nearby 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 formed on the retina. This defect is common after the age of 50 years.
Astigmatism: in astigmatism, a person cannot focus simultaneously on both horizontal and vertical lines. This defect can be corrected by the use of cylindrical lenses.
Sometimes, due to old age, a person may suffer from both myopia and hypermetropia. This defect is known presbyopia. Bifocal lenses, with both concave and convex lenses, are helpful for old people who are suffering form both these defects.
This is how spectacles can help people to see objects clearly. Spectacles have proved to be a great boon for persons with weak vision. In addition, sunglasses with UV filtering or cooling lenses are used for protecting the eyes from the intense rays of the sun. Special glasses are used in UV sunglasses, which prevent the ultraviolet rays of the sun from entering the eyes. Ultraviolet rays given out by the sun are very injurious to the eyes, because they can damage the tissues due to their high energy content.