To fit a contact lens to a patient\'s eye, a keratometer can be used to measure
ID: 1560530 • 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 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 = 30.0 cm and M = 0.0200.
______ mm
Explanation / Answer
p = 30.0 cm
M = 0.0200.
magnification M = -q/p
q = -p*M = -30*0.02
q = -0.6 cm
the lens formula (1/f) = (1/p) + (1/q) = (1/30) + (1/-0.6)
f = -0.61224 cm
the radius of curvature R = 2f = 2*-0.61224 cm
R = -1.22448 cm