Homework Help: Defects of Vision
Inspite of all precautions and proactive action, our eyes may develop some defects due to various reasons. According to homework online experts some of the common optical defects of the eye are:
1. Myopia or short sightedness
2. Hypermetropia or long sightedness
We discuss them here in some detail.
a. Myopia or short sightedness: Myopia or short sightedness is that defect of human eye by virtue of which, the eye can see clearly the objects lying near it, but the far objects cannot be seen distinctly i.e. objects lying beyond a particular distance cannot be seen by the eye. In other words, for a myopic eye, the far point shifts towards the eye. It is no longer at infinity. It is at F.
In fig. 1 parallel rays from infinity are focused on the retina by the normal eye. In fig. the myopic eye focused the || rays from infinity at P in front of the retina. However, according to study daddy the defective eye focuses rays from a point F on the retina. Thus F is the far point of the myopic eye. It cannot see beyond F.
The two possible causes of this defect are:
(i) Increase in size of the eye of the eye ball i.e. distance of retina from the eye lens increases.
(ii) Decrease in focal length of the eye lens, when the eye lens, when the eye is fully relaxed.
To correct a myopic eye of this defect, the person has to use spectacles with a concave lens of suitable focal length. In fact, parallel rays of light from infinity, after refractive through the concave lens, should appear to come from F, the far point of the defective eye. According to studydaddy.com experts this is shown in fig. 1.
Let x be the distance of far point of the myopic eye.
And ƒ be the focal length of the concave lens to be used.
Now, for concave lens, u = ∞, v = -x
As 1/ƒ = (1/v – 1/u)
Or 1/ƒ = (- 1/x – 1/∞)
Or, ƒ = -x
Hence focal length of the concave lens put in front of the eye should be equal to distance of far point of the defective eye.