3. What power of spectacle lens is needed to correct the vision of a nearsighted person whose far point is 30.0 cm? Assume the spectacle (corrective) lens is held 1.50 cm away from the eye by eyeglass frames. OPTICS Defects of vision. Correction of vision 1. Correction lens for near-sightedness: 1 Fmyopia fmyopia Poo Fmyopia – optical power of the lens, prescribed to correct near-sightedness or myopia (diopters), fmyopia – focal length of the lens, prescribed to correct near-sightedness or myopia (m), p = 0 far point of a healthy eye (m), q – desired image distance (m). Lens too strong Eye too long (a) Myopia 2. Correction lens for far-sightedness: Fnyperopia =-+- fnyperopia Po Fnyperopia - optical power of the lens, prescribed to correct far-sightedness or hyperopia (diopters), fnyperopia – focal length of the lens, prescribed to correct far-sightedness or hyperopia (m), Po = 25 cm is a far point of a healthy eye (m), q – desired image distance (m). Lens too weak Eye too short (b) Hyperopia 3. Lens equation: F =-=- +- f– lens focal length (m); F – lens optical power (D); p – distance from the object to the lens or object distance (m); q – distance from the image to the lens or image distance (m). 4. Laser vision correction has progressed rapidly in the last few years. It is the latest and by far the most successful in a series of procedures that correct vision by reshaping the cornea. As noted at the beginning of this section, the cornea accounts for about two-thirds of the power of the eye. Thus, small adjustments of its curvature have the same effect as putting a lens in front of the eye. To a reasonable approximation, the power of multiple lenses placed close together equals the sum of their powers. For example, a concave spectacle lens (for nearsightedness) having P = -3.00 D has the same effect on vision as reducing the power of the eye itself by 3.00 D. So to correct the eye for nearsightedness, the cornea is flattened to reduce its power. Similarly, to correct for farsightedness, the curvature of the cornea is enhanced to increase the power of the eye - the same effect as the positive power spectacle lens used for farsightedness. Laser vision correction uses high intensity electromagnetic radiation to ablate (to remove material from the surface) and reshape the corneal surfaces.