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Question
The near point and the far point for a person are 50 cm and 500 cm, respectively. Calculate the power of the lens the person should wear to read a book held in hand at 25 cm. What maximum distance is clearly visible for the person with this lens on the eye?
Solution
Case (i)-
p = `1/"f"`
`1/"f" = 1/"v" - 1/"u"`
u = - 25 cm
v = 50 cm
p = `- 1/50 - 1/(-25)`
p `= - 1/50 + 1/25`
p = `1/50` cm
p = `1/0.5`
p = 2D
Case (ii-
v = - 500 cm, u = ?,
p = `1/"f" = 2`
f = `1/2`
f = 0.5
f = 50 cm
`1/"f" = 1/"v" - 1/"u"`
`1/50 = - 1/(-500) + 1/"u"`
`1/"u" = 1/500 + 1/50`
`1/"u" = (1 + 10)/(500) = 11/500`
u = `500/11`
u = 45.45 cm
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