An optometrist prescribes corective contact lenses for a nearsighted patient. Th

Question

An optometrist prescribes corective contact lenses for a nearsighted patient. The patient cannot clearly see objects placed more than 2.50m from his eyes. Because of his age, the near point of the patient is 0.50m. Note that the optometrist needs to prescribe bifocal contact lenses. Near point of a normal eye is 0.25m.

In your answers could you explain why you put a positve or negative sign in front of some of the variables in the equation.

a) The power of the upper part of the lenses is _______ diopters (pay attention to sign)

b) The power of the lower part of the lenses is _______ diopters (pay attention to sign)

c) The upper part is a ______ lens and the lower part is a ______ lens.

d) At the store, the technician made a mistake and gave the patient a pair of diveging contact lenses with a focal length of -4.00m. The near point of the patient wearing these contact lenses is

Explanation / Answer

a) -0.4D

A Diverging lens is placed so that when object is placed at infinity image forms at far point of eye

v = -2.5m

u = infinity

F = 1/v + 1/u

F = -1/2.5

F = -0.4D

b) 2D

A Converging lens is placesd so that when object is placed at normal near point we get an image at near point of uncorrected eye.

u = .25m

v = -.5m

F = 1/v + 1/u

F = -2+4

F = 2D

c) Diverging, Converging

d)0.57m

With this lens, an image placed at corrected near point gives image at uncorrected near point

1/f = 1/v + 1/u

-1/4 = -1/.5 + 1/u

1/u = 1.75

u = 0.57 m

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