Nearsighted glasses use concave lenses to correct the blurry distance vision associated with the condition. Nearsightedness, medically known as myopia, is a common refractive error where individuals can see close objects clearly but struggle to focus on things far away. The primary function of the glasses is to adjust how light enters the eye so that distant images become sharp and distinct.
Defining Concave and Convex Shapes
Concave and convex lenses are distinguished by their physical curvature and how they interact with light. A concave lens is characterized by a shape that curves inward, making it thinner in the center and noticeably thicker at the edges. This structure gives the concave lens its property as a diverging lens, meaning it causes parallel light rays passing through it to spread out. Conversely, a convex lens is thickest in the center and tapers to a thinner edge, featuring an outward bulge. This opposite shape causes the convex lens to be a converging lens, which bends light rays inward toward a single focal point.
Understanding How Nearsightedness Affects Focus
Myopia occurs because the eye’s natural focusing power is too strong for its length, causing light to focus incorrectly. This often happens because the eyeball is physically too long (axial myopia) or the cornea is too steeply curved, which increases the eye’s refractive power. When parallel light rays from distant objects enter a myopic eye, they converge to a point in front of the retina, rather than directly on the light-sensitive tissue. Because the light rays have already crossed and begun to spread out again by the time they reach the retina, the distant image is perceived as blurred.
How Concave Lenses Correct Vision
The concave lens is chosen to counteract the eye’s excessive converging power by spreading the light before it enters the eye. As a diverging lens, the concave glass bends incoming parallel light rays outward, effectively pushing the focal point backward within the eye. This initial divergence reduces the overall power of the eye’s optical system. The light is then focused by the eye’s natural components to land precisely on the retina, restoring clear distance vision. The strength of this divergence is measured in negative diopters, with a higher negative number indicating a greater need for the light rays to be spread apart.
The Contrast: What Convex Lenses Are Used For
The convex lens is used to correct farsightedness, or hyperopia. This refractive error occurs when the eye is typically too short, causing light rays to focus at a theoretical point behind the retina. The convex lens functions as a converging lens, bending light inward to increase the eye’s focusing power. This added convergence pulls the focal point forward, placing the image directly onto the retina to correct the blurry vision.