A student is unable to see clearly the words written on the black board placed at a distance of approximately 3 m from him. Name the defect of vision the boy is suffering from. State the possible causes of this defect and explain the method of correcting it.
Myopia is that defect of human eye by virtue of which it can see clearly the objects lying at short distance from it. But the far off objects cannot be seen clearly by the myopic eye.
Causes of Myopia : The two possible causes of this defect are :
Increase in the length of the eyeball, as if distance of retina from the eye lens has increased.
Decrease in focal length of the eye lens when the eye is fully relaxed. This is as if the ciliary muscles holding the eye lens do not relax fully and have some tension.
Correction : The image of a distant object (i.e., at infinity) is formed in front of the retina of eye suffering from myopia as shown in figure. (a)
As the image of the object lying at infinity is not formed on the retina of the eye, so such object can not be seen clearly by the myopic eye. The far point of such an eye is near to the eye as shown in figure (b).
This defect can be corrected by using a concave lens of suitable focal length. So, a man suffering from this defect wears spectacles having concave lens of suitable focal length. The concave lens diverges the rays of light entering the eye from infinity. Hence this lens makes the rays of light appear come from the far point O' of the defective eye as shown in figure (c).
Let x = distance of far point of myopic eye, f = focal length of concave lens to be used. As the object to be seen is at infinity and its image is to be formed at the far point, therefore, u and v = –x. Distance of far point O' from eye lens is taken same as the distance of far point O' from concave lens.
from, 1f=1v−1u
or f = -x
Hence, focal length of concave lens used for correcting the myopic eye is equal to distance of far point of the myopic eye.
Myopia is that defect of human eye by virtue of which it can see clearly the objects lying at short distance from it. But the far off objects cannot be seen clearly by the myopic eye.
Causes of Myopia : The two possible causes of this defect are :
Increase in the length of the eyeball, as if distance of retina from the eye lens has increased.
Decrease in focal length of the eye lens when the eye is fully relaxed. This is as if the ciliary muscles holding the eye lens do not relax fully and have some tension.
Correction : The image of a distant object (i.e., at infinity) is formed in front of the retina of eye suffering from myopia as shown in figure. (a)
As the image of the object lying at infinity is not formed on the retina of the eye, so such object can not be seen clearly by the myopic eye. The far point of such an eye is near to the eye as shown in figure (b).
This defect can be corrected by using a concave lens of suitable focal length. So, a man suffering from this defect wears spectacles having concave lens of suitable focal length. The concave lens diverges the rays of light entering the eye from infinity. Hence this lens makes the rays of light appear come from the far point O' of the defective eye as shown in figure (c).
Let x = distance of far point of myopic eye, f = focal length of concave lens to be used. As the object to be seen is at infinity and its image is to be formed at the far point, therefore, u and v = –x. Distance of far point O' from eye lens is taken same as the distance of far point O' from concave lens.
from, 1f=1v−1u
or f = -x
Hence, focal length of concave lens used for correcting the myopic eye is equal to distance of far point of the myopic eye.
