Final answer:
The Ray Deflection principle is used in a subjective optometer to determine where light rays converge on the retina, helping to measure refractive error and estimate the necessary corrective lens power. It is also applied in other optical instruments, such as keratometers, to measure the curvature of the cornea for contact lens fitting.
Step-by-step explanation:
The Ray Deflection principle is employed in an subjective optometer to measure a person's refractive error by determining the focus of light rays on the retina. By adjusting the optometer, one can ascertain the point at which the rays converge inside the eye, which indicates whether the person is nearsighted or farsighted. The device makes use of lenses and may involve a convex air-glass interface to refract the light rays. By using a ruler to estimate the radius of curvature, one can deduce the optical power needed to correct the patient's vision.
This ties into the wave characteristics of light, as outlined by Huygens's principle, where the wavelength of light is considered. In other optical devices, such as a telescope with convex and concave lenses, calculations would be made for the location and size of the image produced, factoring in the focal lengths of the lenses and the size and location of the object to verify magnification.
Similarly, in medical instruments like a keratometer, which measures the curvature of the cornea, light is reflected from the cornea and the measurement of the magnification of this reflection helps determine the radius of curvature, critical for fitting contact lenses correctly.