The observation that a person’s eyes look distorted, either larger or smaller, when viewed through their eyeglasses is a direct result of physics, not an optical illusion. Corrective lenses are designed to manipulate light, which inevitably alters the apparent size and position of the wearer’s eyes behind the lens. This effect is rooted in the fundamental principles of optics and how different lens shapes bend light as it travels toward the observer.
The Core Principle of Light Refraction
Eyeglasses correct refractive errors—issues with how the eye focuses light onto the retina—by using the principle of refraction. Refraction is the bending of light as it passes from one medium, like air, into another, like the lens material. The lens shape is engineered to change the light’s direction so it lands correctly on the retina for clear vision.
When an observer looks through the glasses, light rays coming from the wearer’s eye are bent by the corrective lens before reaching the observer’s eye. This pre-bending causes visible distortion because the observer’s brain interprets the eye’s size and location based on the altered light path. Whether the light is converged or diverged determines if the eye appears magnified or minimized.
Why Lenses for Farsightedness Magnify the Eyes
Lenses correcting farsightedness—where close objects are blurry—are convex lenses. Farsightedness means the eye focuses images behind the retina, requiring light to be converged more strongly. Convex lenses are thicker in the center, thinner at the edges, and are also called plus lenses because they have a positive power that converges light rays.
When light passes through a convex lens, the outward-curving surfaces bend the rays inward toward a central focal point. This convergence makes the wearer’s eye appear larger and closer to the observer, similar to a magnifying glass. A greater degree of farsightedness requires a thicker lens center, resulting in a more pronounced magnification effect on the eyes.
Why Lenses for Nearsightedness Minimize the Eyes
Conversely, lenses prescribed for nearsightedness—characterized by blurry distance vision—are concave lenses. Nearsightedness causes the image to focus in front of the retina, requiring light rays to be spread out before entering the eye. Concave lenses, also called minus lenses, have a negative power and are thinner in the center and thicker at the edges.
The inward-curving surfaces of the concave lens cause light rays to diverge, or spread out, as they pass through. This divergence pushes the focus point backward onto the retina, correcting the vision error. When an observer looks through a concave lens, the spreading of the light rays makes the eye appear smaller and farther away, creating a noticeable minification effect.