Final answer:
The diopter on the ophthalmoscope measures the refractive power of the eye and helps determine appropriate corrective lenses.
Step-by-step explanation:
The diopter on the ophthalmoscope is used to measure the refractive power of the eye. It measures the ability of the eye to focus light on the retina. The diopter is a unit of measurement that represents the reciprocal of the focal length of a lens. In the case of the ophthalmoscope, the diopter helps the examiner to determine the appropriate corrective lenses for a patient with vision problems.