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Ray Optics and Optical Instruments — Physics STD 12 Science — Question

Rajasthan BoardEnglish MediumSTD 12 SciencePhysicsRay Optics and Optical Instruments5 Marks
Question
Mention the defects of human vision and describe the method to remove them.
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Answer

The eye, as any other organ can suffer from problems or defects. we will consider four kinds of defects of vision, viz., short-sightedness or myopia; long-sighedness or hypermetropia; presbyopia; and astigmatism.
Short sightedness or Myopiya : This defect is also known as nearsightedness. A short-sighted person cannot see objects placed beyond a certain distance from the eye. This distance is the short-sighted eye's far point. Nearsightedness can occur because (a) the eyeball is elongated, i.e., the distance between the lens and then retina is greater than normal, or (b) the maximum focal length of the lens (when the eye is relaxed) is less than usual.
Image
The result is that a parallel beam of light froma distant object converges short of the retina [Figure (a)]. However, a divergent beam from any point closer than the far point converges on the retina [Figure (b)]. This defect can be corrected by using a concave lens of appropriate focal length. The lens should be such that parallel rays from a distant object appear to come from the far point of the eye after refraction from it [(c)]. Suppose the focal length of the lens to be used is $f$. Then $u$ $=-\infty$ and $v=-d$ (the distance of the far point O from the concave lens), and from the lens formula,
$\frac{1}{-d}-\frac{1}{\infty}=\frac{1}{f} \Rightarrow f=-d$ ...(i)
The negative sign of the focal length shows that the lens is concave. The focal length of the lens should be equal to the distance of the far point of the eye.
Long-sightedness or hypermetropia : The defect is also known as farsightedness or hypermetropia. The eye, in this case, cannot see objects within a certain distance from the eye. The minimum distance beyond which it can see clearly without straining is its near point. The possible causes of this defect are (a) the shortening of th eyeball, i.e., the distance between the lens and the retina is less than normal, or (2) the focal length of the lens is greater than usual.
Consequently, rays from any point, N', which is closer than the near point, N, converge behind the retina [Figure (a)], while rays from all points, N', beyond the near point converge on the retina [Figure (b)]. This defect can be corrected by using a convex lens of the appropriate focal length to make the divergent rays from an object placed at the normal near point, N, converge on the retina after refraction by the convex lens. Then rays from the object appear to come from the point N', the near point of the defective eye, and can be seen clearly [Figure (c)].
Image
Suppose the focal length of the convex lens used is $f$. Then $u =- D$ and $v=-d$ the distance of the near point N' from the convex lens.) Thus, from the lens formula,
$\frac{1}{f}=\frac{1}{-d}-\frac{1}{- D }=\frac{d- D }{ D d} \Rightarrow f=\frac{ D d}{d- D }$ ...(ii)
In equation (ii), as d > D the focal length is positive, which shows that the lens used is convex. Since the image of an object placed at the normal near point is formed at the near point N' of the defective eye, the image of any object situated between N and N' wil be formed beyond the near point of the defective eye and seen clearly due to the eye's accomodating power.
Presbyopia : In old age, the understanding capacity of the eye lens decreases. Therefore, neither objects that are very close nor that are very far away are clearly visible. This defect of the eye is called presbyopia. This defect can be removed by using bifocal lenses.
Astigmatism : An eye with astimatism cannot see horizontal and vertical lines located at equal distances with equal clearity. This defect arises due to unequal curvatures of the vertical or horizontal parts of the cornea or retina. This defect is rectified by the use of cylindrical lenses.

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