Passage For a person with perfect vision, light from an object is properly refracted by the eye lens to converge on a single point at the retina, forming a clear image of the object. Vision defects result from eye shape abnormalities or errors in the refractive power of the eye lens. Myopia (nearsightedness) occurs when light from a distant object is incorrectly focused in front of the retina. Hyperopia (farsightedness) occurs when light rays from a nearby object are focused beyond the retina.Many optical techniques are available to measure the refractive error of an individual to determine the necessary correction. Photorefraction is a photographic technique often used with young children because it does not require the individual to be still for a lengthy duration. When the patient is looking at the camera, a flash photograph is taken of the eye to determine the amount of light that is reflected off the retina and captured by the camera lens.In healthy eyes, all the light from the flash that enters the eye is reflected off the retina and returns back to the camera's light source. Because the camera lens does not receive this light, the pupil is completely dark in the resulting image. A myopic eye cannot properly focus the light at the retina. Due to the geometry of the eye and its lens, some of the light is reflected to the top portion of the camera lens. The camera captures an image of a pupil with a crescent of light at the top. In a hyperopic eye, the crescent appears at the bottom of the pupil. Ray diagrams for photorefraction are shown in Figure 1. Figure 1 Paths of light in photorefraction for different eyes: (A) Healthy, (B) Myopic, and (C) Hyperopic. HC. Howland, "Optics of photorefraction: orthogonal and isotropic methods." ©1983 Optical Society of America. -A patient's eye lens has a focal length of 2 cm. When the patient wears a pair of prescription eyeglasses with an optical power of −4 D, the combined strength of the lenses is approximately:

Question

Quizplus · Accepted Answer

11ecba2d_12d1_64a5_a3bf_9740d839f899_MD0008_00 Optical power, or lens strength, is given in units of diopters (D). A diopter is equal to an inverse meter (m⁻¹). The strength of a lens is calculated as the inverse of its focal length (f): 11ecba2d_12d1_8bb6_a3bf_9d044cd8fa8b_MD0008_00 The patient's eye lens strength is determined by converting the given focal length from centimeters to meters (2 cm = 0.02 m) and inverting this value: 11ecba2d_12d1_8bb7_a3bf_07bc8e62b251_MD0008_00 The strength of the eye lens is 50 D (Choice C). The lens strength (and focal length) of a converging lens is positive. The eyeglasses' optical power is given as −4D, which means that it is a diverging lens. For multiple lenses in close proximity, their combined lens strength is approximated as the sum of their individual lens strengths: 11ecba2d_12d1_8bb8_a3bf_7791e4a7a2f3_MD0008_00 These eyeglasses have a negative prescription because they are designed to effectively lower the patient's lens strength from 50 D (Choice D). The approximate combined lens strength is: 11ecba2d_12d1_8bb9_a3bf_6701a2d03e0d_MD0008_00 Educational objective: The optical power (or strength) of a lens is equal to the inverse of its focal length in meters (S = 1/f). The value is positive for converging lenses and negative for diverging lenses. The combined lens strength of a multi-lens system is approximated as the sum of the strengths of each individual lens.

Passage For a Person with Perfect Vision, Light from an Object