To fit a contact lens to a patient\'s eye, a keratometer can be used to measure
ID: 1466009 • Letter: T
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 Mof 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 = 35.0 cm and M = 0.0130.
answer in: mm
Explanation / Answer
We use the lens eqn 1/p + 1/q = 2/R and M = -q/p...Here p = 35.0cm
So q = -M*p = -0.013*35.0cm = -0.455cm
Now 2/R = (1/35 + 1/(-0.455)) = -2.169
Therefore R = 2/-2.169 = -0.922cm
The negative sign tells us that the surface is convex.