1 A particular nearsighted patient cant see objects clearly

1. A particular nearsighted patient can\'t see objects clearly when they are beyond 25 cm (the far point of the eye). (a) What focal length should the prescribed contact lens have to correct this problem? (b) Find the power of the lens, in diopters. Neglect the distance between the eye and the corrective lens. SOLUTION: (a) f = -25.0 cm (b) p = -4.00 diopters

2. (a) What power lens would you prescribe for a patient with a far point of 30.0 cm? Neglect the eye–lens distance. SOLUTION: -3.33 diopters

(b) Repeat, assuming an eye-corrective lens distance of 2.00 cm. _____________ diopters (FYI the answer is neither 50 nor 46.66)

Solution

a) let f be the focal length.

for the patient, object distance = o = infinity ; and image distance = i = - 25 cm

from lens equation we know that,

1/f = 1/i + 1/o

1/f = 1/(-25) + 1/infinity => f = -25 cm

Hence, f = -25 cm

b)We know that, power is

P = 1/f = 1/(-0.25m) = -4 diopters

Hence, P = -4 diopters.