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 keratometercan be used to measure the curvature of the cornea-the frontsurface of the eye. This instrument places an illuminated object ofknown size at a known distance p from the cornea, whichthen reflects some light from the object, forming an image of it.The magnification M of the image is measured by using asmall viewing telescope that allows a comparison of the imageformed by the cornea with a second calibrated image projected intothe field of view by a prism arrangement. Determine the radius ofcurvature of the cornea when p = 25.0 cm and M = 0.0100.
mm

Explanation / Answer

Given :             Object distance (p) = 25.0   cm              Magnification (M)   = - q / p   = 0.0100                           or   - q     =    0.0100 * p                                            =   0.0100 * 25.0 cm                                            =   0.25   cm      or   Image distance( q) =   - 0.25 cm We know that :                    1 / f    =   1/ p   + 1 / q                              =   1 / 25   - 1 / 0.25           Focal length (f) =  --------- cm Radius of curvature (R) =   2 * focallength (f)                                      =  ---------   cm Solve the above. Hope this helps u!

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