How do we see Clearly with Spectacles?
What are Spectacles and Lenses
All men women and children, who have weak eye-sight, use spectacles. Scientist have developed spectacles that help in seeing both the near distant object clearly. The history of spectacles was started some 700 years ago. In 1266, Roger Bacon of England used a pieceof glass to magnify the words written in a book. Spactacles are shown on the eyes of one Cardinal Ugon’s portrait made in 1352. In 1784 Benjamin Franklin brought wonders in the feildof spectacles by making bifocal lenses. How does it work the light ray enter your 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 retina.The light rays coming from any object make in inverted image of object on the retina with the help of this lens. This is carried to the brain by the optic nerve. It becomes erected there. This is how we see an object.
Sometimes, the eyes develop some defacts due to which the image of the object is formed before or behind the retina. and is blurred. Persons having such eyes defacts need the assistance of spactacles.
The defacts of the eyes are mainly of three types:-
1. Myopia:- Persons suffering from this defect can see the nearby object clearly, but the distant object appear blurred. (Solution- Spectacles with concave lens)
2. Hypermetropia:- Persons suffering from this defact cannot see nearby objects clearly but can see distant object clearly. (Solution- Spectacles with convex lens)
3. Astigmatism:- Due to this defact in the eyes, one cannot see with clearity the horizontal and vertical lines simultaneously. (Solution- Spactacles with cylindrical lenses)