a person has myopia and so wears spectacles with power -5D. on calculation his far point comes out to be 20 cm. (i) how can the far point of a person be less than his near point. (ii) what is the range of visibility of such a person.???
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Well the person is not a normal person, because the normal human eye has near point 25cm. Myopia is the eye defect in which the person is able to see the nearby objects but not able to see distinct object clearly. If the person is suffering from myopia defect with far point 20cm then the person has near point less than that of 20cm, as myopia defect enabled the person to see nearby objects then the person is able to see the nearby object kept within 20cm !
(ii) Given ;
P = -5D
u = -20 cm
v = ?
P = 1/f
f = 1/-5
f = -1/5 cm or -0.2 cm
By using lens formula;
1/f = 1/v - 1/u
-5 = 1/v + 1/20
-1/v = 1/20 + 5
1/v = - 101/20
v = -5.05
Therefore the near point of the persona eye is 5.05 cm
Hence the range of visuality of a person is from 5.05cm to 20 cm
(ii) Given ;
P = -5D
u = -20 cm
v = ?
P = 1/f
f = 1/-5
f = -1/5 cm or -0.2 cm
By using lens formula;
1/f = 1/v - 1/u
-5 = 1/v + 1/20
-1/v = 1/20 + 5
1/v = - 101/20
v = -5.05
Therefore the near point of the persona eye is 5.05 cm
Hence the range of visuality of a person is from 5.05cm to 20 cm
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