The far point of an eye is 185 cm. A corrective lens is to be used to allow this eye to focus clearly on objects a great distance away. What should be the focal length of this lens?
What is the power of the needed corrective lens in diopters?
a) consider an object at the required corrected near point - inf
The required image distance is at 105 cm, since this is the nearest
that the eye can focus clearly. With this information we can use
this formula -
1/f = 1/v + 1/u where f is the focal length of the lens, v is the
image distance, and u is the object distance. Because the image is
a virtual one, v is given a negative sign.
1/f = - 1/105
f = 1.05m
b) The power in diopters is the reciprocal of the focal length in
metres; this is 0.952 diopters
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