It's estimated that around half of the world's adult population have common conditions affecting their vision, such as nearsightedness, farsightedness, astigmatism, and the age-related condition presbyopia. Fortunately, we live in a time when these issues can easily be resolved with the use of corrective glasses or contact lenses - although it's worth noting that there are thought to be over one billion people around the globe who require corrective lenses but don't have access to them.
To understand exactly how eyeglasses correct vision, it's crucial to first gain an awareness of the physics of vision and how the different parts of the eye work together to help create the miracle of sight.
The Physics of Vision
The eye is a wondrous optical instrument. It can focus on objects at a variety of distances and automatically adjust to a range of light intensities. And to get even more scientific about it, the eye is sensitive to continuous ranges of electromagnetic waves from below 400 nm to around 650 nm in wavelength. Impressive.
The transparent layer at the front of the eye is the cornea, and this acts as a 'fixed focus' lens. The lens of the eye itself is attached to the ciliary muscles and is flexible. These muscles alter the eye lens' thickness which, in turn, changes its optical power. By doing so, the lens can form an image on the retina of any object within a range of distances.
The iris is the colored part of the eye, and its job is to control the amount of light that enters the eye. In bright light, the iris expands to make the eye pupil narrower, meaning that less light can pass through it, while in dim light, the iris contracts, serving to dilate the pupil and therefore enable more light to pass into the pupil.
Correcting Near and Far-Sightedness
For those with near or far-sightedness, eyeglasses can be worn to correct the vision. Nearsightedness (myopia) denotes the inability to see distant objects clearly, while nearer objects can be clearly seen. This happens due to the myopic eye converging rays from a distant object in front of the retina rather than onto it, which results in the image being blurry.
Therefore, to resolve nearsightedness, a concave spectacle lens is placed in front of the eye that serves to correct this, converging the rays onto the retina, thereby making the object clear. The lens effectively creates an image that's closer to the eye than the object is in reality.
Those with farsightedness (hyperopia) are able to see things at a distance more clearly than objects closer up due to the eye converging rays from nearby objects behind the retina instead of onto it. Convex eyeglass lenses are used to correct hyperopia.
How Do Eyeglasses Correct Astigmatism?
Astigmatism is a common eye condition in which the eye (usually the clear front part of the eyeball or the cornea) isn't completely round. This causes light entering the eye to be bent unevenly, making an object look blurry and wavy. Many people with myopia or hyperopia also have astigmatism.
Corrective lenses can be used to correct astigmatism. The lenses are curved, acting to counteract the shape of the lens or cornea that's causing the problem. However, those with severe astigmatism may find that the strength of the lenses causes the peripheral vision to have a 'tilted' appearance. This effect often goes away by itself as the wearer gets used to their lenses - if it doesn't, then rigid gas-permeable contact lenses tend to be an effective solution to astigmatism.
Using Eyeglasses to Correct Presbyopia
Presbyopia is an age-related condition that causes the eyes to lose their ability to focus on nearby objects. Corrective glasses can also be worn to resolve this problem - typically, bifocal or multifocal lenses can be worn that allow the wearer to see close-up objects without affecting their ability to focus on other objects that are further away.
Multifocal lenses incorporate two or more vision-correcting prescriptions. As well as bifocals, trifocals, or progressive lenses could also be an option to correct the vision.
The Future of Eyeglasses
The future of glasses is an exciting one, with new developments in corrective lenses now coming thick and fast. Nano-drops, smart glasses, and the visual cortical prosthesis system (which can potentially restore sight to the blind) are just the latest innovations. Auto-focusing eyewear that works by deploying eye-tracking software is currently in the research stages, promising new advances just around the corner.
