Nearsightedness, medically termed myopia, is a common refractive error where light entering the eye focuses in front of the retina instead of precisely on its surface. This results in distant objects appearing blurry, while things close up typically remain clear. Myopia often occurs because the eyeball is slightly too long, or the cornea is too curved, giving the eye too much focusing power. Corrective lenses counteract this excess focusing power by shifting the focal point backward onto the retina, restoring clear distance vision.
Identifying the Corrective Lens
The lens used to correct nearsightedness is known as a concave lens. This lens type is also commonly referred to as a diverging lens because of how it interacts with light rays. A key physical characteristic of a concave lens is its shape, which is thinner in the center and thicker toward the edges.
The Physics of Myopia Correction
The concave lens works by introducing a controlled amount of divergence to the incoming light rays before they enter the eye’s natural focusing system. In a myopic eye, the light is converged too strongly, causing the image to form prematurely. The corrective lens gently spreads the light rays apart. This divergence effectively reduces the overall focusing power of the combined system—the corrective lens and the eye itself.
By spreading the light, the focal point is pushed further back within the eye. The light rays then enter the cornea and crystalline lens, which perform their usual focusing. The net result of this two-step process is that the final focal point lands exactly on the retina. The stronger the degree of nearsightedness, the more divergence is needed, which translates to a stronger minus-powered lens.
Deciphering Your Prescription
The power of a corrective lens for nearsightedness is measured in a unit called diopters (D). The number indicating the lens power will always be preceded by a negative, or minus, sign (–). This negative sign denotes that a diverging lens is required to correct the refractive error. The further the numerical value is from zero, the stronger the lens power and the greater the degree of myopia.
The prescription often contains other values, such as Cylinder (CYL) and Axis, which account for astigmatism. Astigmatism is a condition where the cornea is shaped more like a football than a sphere, causing light to focus unevenly. The CYL number indicates the strength of the corrective power needed for this irregularity. The Axis specifies the orientation of the astigmatism on the eye, given as a degree between 1 and 180.
Corrective Lens Options
The concave lens needed to correct myopia can be delivered through two primary means: spectacles (glasses) or contact lenses.
Spectacles
Spectacles are often the first line of correction because they are easy to maintain, simple to wear and remove, and require no direct contact with the eye. They also offer an effective protective barrier against environmental elements like dust.
Contact Lenses
Contact lenses, which sit directly on the surface of the eye, provide a more natural, uninterrupted field of vision compared to glasses. They move with the eye, which eliminates the peripheral distortions that can sometimes occur with spectacle lenses. However, contact lenses require a more rigorous cleaning and maintenance routine to prevent the risk of eye irritation or infection. For children with rapidly progressing myopia, specialized contact lenses have also been shown to be more effective at slowing the rate of progression than standard spectacle lenses.