|  (Some friends have sent mails to me, requesting me to start some discussions on ‘scientific’ topics, which any body can understand & respond, but at the same time, one says: “IT IS NOT YOUR CLASS, WE ARE NOT HERE TO STUDY”. Any way, let me continue my efforts in sharing of knowledge. DR.B.SARAF Ph.D.)
Normal human eye can see ‘clearly’ objects lying from about 25cm to infinity, and all colors. Any deviation from these is called ‘defects of vision’. Our eye is a spherical box, having a variable power ‘convex’ lens at the front and a screen called ‘retina’ on the back. This can focus the image of an object in front of it from about 25cm to a very large distance, on the ‘retina’. If due to any reason,(eye lens becomes thick or eyeball becomes longer) the image gets focused, not on the retina, but a little earlier, then one ‘cannot’ see distinctly, the ‘far’ objects but he may not have problem in seeing ‘near’ objects. This defect is called ‘near vision’. Similarly, if the image is focused, a little beyond the retina, (eye lens becomes thin or eye ball becomes smaller) then he can see distinctly, the ‘near’ objects, while, he may have problem in seeing ‘far’ objects, this is called ‘far vision’. A lens has a fixed power. The distance from the ‘centre’ of the lens, to where the image is formed is called ‘focal length’. This can be changed by keeping another lens near it or by changing the shape of the lens by grinding. Keeping one ‘convex lens’ near another ‘convex lens’ will decrease the effective ‘focal length’, but keeping a ‘concave lens’ near a ‘convex lens’ will increase the ‘effective’ focal length. Convex lens power is said to be +ve and ‘cocave’ lens are said to have –ve power. The ‘convex’ lens of the eye has the capability to ‘change’ its focal length ‘automatically’ by becoming thicker or thinner, with the help of muscles, to view objects in the range specified. This is called ‘power of accommodation’, but there is a limit, even after becoming fully thick, it cannot see clearly, objects lying closer than 10 inches or 25 cms. This is called, ‘least distance of distinct vision’.
Now, things are easy to understand, give a spectacle to a ‘far sighted’ person, fitted with a pair of ‘convex’ lens of suitable power, the focal length will be reduced, and the image will be rightly formed on the retina, he can now see distinctly the near objects too. This happens to persons, around age 40, because the ‘muscles’ gradually fail to change the shape of the eye lens. So with advancing age, you need more and more thick ‘convex’ lens. This problem should increase with age. Similarly, give a spectacle with concave lenses to a ‘near sighted’ person, the focal length will increase & the image will be rightly formed on the retina. This defect is likely to decrease with age. Inability to see all colors is called, ‘color vision’. (WE HAVE DISCUSSED, ONLY THE SIMPLE DEFECTS).
Now, tell me (1)What defect you have? (2)What is the power of your lens? (3)Do both lens have same power? (4)How very old persons, who used high power convex lens earlier, are now able to read ‘without’ lens, without any surgery now??
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