Chapter : 5. Human Eye & Colourful World
Long sightedness or hypermetropia
2. Long sightedness or hypermetropia
Symptoms : With this defect eye cannot see clearly within a certain distance. It means that the near point of the defective eye has shifted from 25 cm to some more distance behind (away).
Reason : It is so because the image of near objects is formed behind the retina. It is shown in fig.
Hypermetropic eye vision.
Causes :
(i) The eye lens may be thinner (less converging) than the normal eye lens.
(ii) The eye ball may be oval distance between lens and retina becomes less than that for normal eye.
Fig. Oval eye.
Correction : The deficiency in converging power of eye lens is compensated by using a convex (Converging) lens of proper power (focal length) as shown in fig.
Fig : Hypermetropia corrected by a convex lens.
Calculation : Let distance of near point N' from eye = y. Then, for lens to be used
u = –D, v = –y, f = ?
From lens formula,
(Which is positive, y > D)
This is required expression for the focal length of the convex lens to be used.
Symptoms : With this defect eye cannot see clearly within a certain distance. It means that the near point of the defective eye has shifted from 25 cm to some more distance behind (away).
Reason : It is so because the image of near objects is formed behind the retina. It is shown in fig.
Hypermetropic eye vision.
Causes :
(i) The eye lens may be thinner (less converging) than the normal eye lens.
(ii) The eye ball may be oval distance between lens and retina becomes less than that for normal eye.
Fig. Oval eye.
Correction : The deficiency in converging power of eye lens is compensated by using a convex (Converging) lens of proper power (focal length) as shown in fig.
Fig : Hypermetropia corrected by a convex lens.
Calculation : Let distance of near point N' from eye = y. Then, for lens to be used
u = –D, v = –y, f = ?
From lens formula,
(Which is positive, y > D)This is required expression for the focal length of the convex lens to be used.
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