To fit a contact lens to a patient\'s eye, a keratometer can be used to measure

Question

To fit a contact lens to a patient's eye, a keratometer can be  used to measure the curvature of the cornea-the front surface of the  eye. This instrument places an illuminated object of known size at a  known distance p from the cornea, which then reflects some light from the object, forming an image of it. The magnification M  of the image is measured by using a small viewing telescope that allows  a comparison of the image formed by the cornea with a second calibrated  image projected into the field of view by a prism arrangement.  Determine the radius of curvature of the cornea when p = 25.0 cm and M = 0.0160.

Explanation / Answer

We use the lens eqn 1/s + 1/s' = 2/R and M = -s'/s...Here s = 30.0cm

So s' = -M*s = -0.013*30.0cm = -0.39cm

Now 2/R = (1/30 + 1/(-0.39)) = -2.531

Therefore R = 2/-2.531 = -0.790cm

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