Vision Defects and Corrections

Myopia (Short-Sightedness) and Its Correction

• People with myopia can see nearby objects clearly, but distant objects appear blurred.
• This happens for two biological reasons: the structural shape of the eyeball is too long, or the lens is too thick and curved, meaning it refracts light strongly.
• Because the refractive power is too high, light rays from distant objects converge and focus at a point in front of the retina, crossing and spreading out to create a blur.
• To correct myopia, a concave lens is placed in front of the eye.
• This lens is thinner in the middle and causes light rays to diverge (spread out) slightly before they enter the eye, moving the focal point further back so it falls exactly on the retina.

Hyperopia (Long-Sightedness) and Its Correction

• People with hyperopia can see distant objects clearly, but nearby objects appear blurred.
• This occurs when the eyeball is too short, or the lens is too flat, thin, or stiff, meaning it provides insufficient refractive power and refracts light less.
• Consequently, light rays from near objects do not converge quickly enough and focus at a point behind the retina.
• Hyperopia is corrected using a convex lens.
• This lens is thicker in the middle and causes light rays to converge (bend inwards) before entering the eye, providing the extra refractive power needed to move the focal point forward onto the retina.

Interpreting Ray Diagrams

• In an exam, you must analyse ray diagrams to identify uncorrected and corrected vision defects by locating the focal point.
• In uncorrected myopia, parallel light rays from a distant object meet and cross inside the vitreous humour, before reaching the back wall of the eye.
• In corrected myopia, a lens with "caved-in" sides is drawn; the rays spread apart slightly before hitting the eye, then converge exactly on the retina line.
• In uncorrected hyperopia, rays from a near point source are drawn hitting the retina while still separate, as if they would meet in the space behind the eyeball.
• In corrected hyperopia, an oval-shaped lens causes the rays to bend slightly inward first, allowing them to finally converge precisely on the retina.

Modern Technologies for Vision Correction

• Traditional spectacle lenses (glasses) remain the most common correction, but modern treatments offer alternatives.
• Contact lenses are thin lenses (available as hard or soft versions) that sit directly on the surface of the eye to alter refraction.
• Laser surgery can permanently reshape the cornea to change how it refracts light, such as reducing its thickness to treat myopia.
• For severe defects, replacement artificial lenses involve surgery to remove the natural lens and replace it with a clear plastic artificial one.