When a person with normal visual acuity stands 20 feet from a Snellen eye chart (so that accommodation is not a factor influencing acuity), the line of letters marked "20/20" can be read. If a person has myopia (nearsightedness), this line will appear blurred because the image will be brought to a focus in front of the retina. This is usually due to the fact that the eyeball is too long. Myopia is corrected by glasses with concave lenses that cause the light rays to diverge, so that the point of focus is farther from the lens and is thus pushed back to the retina (fig. 10.34).
If the eyeball is too short, the line marked "20/20" will appear blurred because the focal length of the lens is longer than the distance to the retina. Thus, the focus of the image would have been behind the retina, and the object will have to be placed farther from the eyes to be seen clearly. This condition is called hyperopia (farsightedness). Hyperopia is corrected by glasses with convex lenses that increase the convergence of light, so that the point of focus is brought closer to the lens and falls on the retina.
Was this article helpful?
This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.