Common Eye Defects and Their Correction

Challenges to clear

• Diagnose all four patients.
• Diagnose the classmate who reads a book easily but not the board.

The task

Four people describe what they can and cannot see. Work out the defect and its correction.

What this lesson covers

• A classmate copies from her textbook without any difficulty, but she cannot read what is written on the board unless she moves to the front row. Best choice: Myopia — corrected with a concave (diverging) lens. — Near objects clear, distant objects blurred is exactly myopia. The eyeball is lengthened from front to back or the lens is too curved, so the image of distant objects falls in front of the retina. A concave lens diverges the rays before they strike the eye lens, and its power is marked with a minus sign.
• Your uncle holds the newspaper at arm's length to read it, though he reads distant street signs perfectly. He is 52 and jokes that his arms are getting too short. Best choice: Presbyopia — the lens has lost its flexibility with age; a convex lens corrects it. — Presbyopia affects older people whose lens can no longer become rounded enough for near vision, giving a kind of far-sightedness. A convex converging lens supplies the extra convergence the stiffened lens can no longer provide.
• Looking with one eye at a chart of lines drawn in many directions, a boy finds one set of lines dark and sharp while the set at right angles to it looks faint and blurred. Best choice: Astigmatism — caused by uneven curvature of the cornea and corrected by cylindrical lenses. — An unevenly curved cornea focuses one direction properly and blurs the direction at right angles to it, which is why some parts of an object are in focus while others are not. Cylindrical lenses compensate for the uneven curvature.
• An elderly neighbour says her vision has slowly clouded over. A doctor shines a light into her eye and finds the lens itself has turned opaque and whitish. Best choice: Cataract — the lens is removed surgically and replaced by an implanted plastic lens or highly convex spectacles. — Cataract is precisely this: the lens turns opaque and vision is cut down, even to total blindness. Once the lens is removed the eye has lost its converging power, so a highly convex spectacle lens or a small plastic implant must replace it.

Hint

Match the lens to the defect: myopia takes a CONCAVE (minus) lens; hyperopia and presbyopia take a CONVEX (plus) lens; astigmatism takes a cylindrical one. Myopia means the eyeball is too LONG, hyperopia that it is too SHORT.