Question:
A myopic adult has a far point at 0.1 m. His power of accomodation is 4 diopters.
(i) What power lenses are required to see distant objects?
(ii) What is his near point without glasses?
(iii) What is his near point with glasses? (Take the image distance from the lens of the eye to the retina to be 2 cm.)
Solution:
(i) Power lenses are required to see distant objects
1/f = 1/v – 1/u
1/f = 1/10
f = -10 cm = -0.1 m
P = 1/f
P = 1/(-0.1)
P = -10 diopter
(ii) When no corrective lens used
Pn = Pf + Pa
u = -10 cm = -0.1 m
v = 2 cm = 0.02 m
1/f = 1/v – 1/u = 60
Pn = Pf + Pa = 64 D
u = xn
xn = 1/14 m = 100/14 cm = 7 cm
(iii) When used corrective lens
u = infinity
v = 2 cm = 0.02 m
1/f = 1/v – 1/u
P = 50
Pn = 50 + 0
Pn = 50 D
u = xnm
xm = ¼ m = 0.25 m