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?

Answer 1

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