How do Glasses Work?

Glasses are worn by people in every corner of the world. Learn about how these seemingly magical pieces of glass let people see

About 79% of the American population needs a form of visual aid. This can include prescription eyeglasses, contact lenses, or laser corrective surgery. Of these 198 million people, a majority of about 84% use eyeglasses! This proportion increases as people get older, and most senior citizens use some form of vision aid. The spectacular invention of eyeglasses, also called glasses or spectacles, helps millions of people every day. But how exactly do they work?

Table of Contents

Who invented glasses?

It is unclear exactly when and where glasses originated, but its clear many people all over the world had similar ideas. The oldest recorded use was by the Ancient Roman Emperor Nero, who would hold an emerald up to his eye when watching his gladiators fight in the Colosseum. Marco Polo reported seeing people in China holding two pieces of glass fixed to a frame up to their eyes. Modern eyeglasses, which rest on the ears, are thought to have been invented by Salvino D’Armati in the thirteenth Century. His grave even bears the inscription “inventor of spectacles.” At this time, glasses were so expensive that they were specially bequeathed in people’s wills!

 

Glasses largely remained the same until the 18th Century. Benjamin Franklin is popularly credited with the invention of bifocals, but there is evidence that artists Joshua Reynolds and Benjamin West jointly discovered them at around the same time. Before bifocals, people who were both near-sighted and far-sighted needed to switch between two pairs of glasses when viewing objects at different distances.

A nineteenth century pair of glasses
A nineteenth century pair of glasses, Credit: Wikimedia/Unknown author

During the early 20th Century, Owen Aves invented the progressive lens. While bifocals were revolutionary, they still could not work on objects at an intermediate distance. The progressive lens solved this issue and is now one of the most popular types of glasses.

What is a lens?

Lenses work on the principle of refraction. When light touches or falls on any surface, it can get reflected, refracted, or absorbed. When light gets reflected, it goes back the way it came, as with a mirror. When light is absorbed, none passes through. When refraction occurs, the path of the light changes.

 

Refraction occurs due to a change in the speed of light. The more solid the medium, the slower the light travels. Refraction can be viewed using a simple experiment. Place a pencil in a glass half-full of water; the pencil will appear as if bent at the border of the water and air because light moves more slowly through water than air.

 

A lens is just a thin piece of glass. Light slows down when it moves from air to solid glass, and the path changes. Light can either get reflected outward (diverge) or inward (converge), leading to the classification of two main kinds of lenses: diverging and converging. The change in the path of light depends on the shape of the glass.

 

Lenses are named after the Latin word for lentil: lens. In fact, a convex lens looks just like a lentil! It has one flat end, and one outward curving end. Convex lenses converge light. Concave lenses curve inward, just like caves. These lenses make light diverge. In other words, convex lenses are converging lenses and concave lenses are diverging lenses.

Light moving through a lens, where the new path depends on the shape of the lens.
Light moving through a lens, where the new path depends on the shape of the lens, Credit; Wikimedia/Henrik

When light rays meet either in front of or behind a lens, an image forms. This image is a representation of whatever object sits in front of the lens. Depending on where the object is, the image might be right side up, upside down, zoomed in, or even shrunken down.

How does the human eye work?

The human eye has an incredible number of parts, and is very intricate. For the eye to function right, every single piece needs to work correctly! When light hits the eye, the first part it interacts with is the pupil. The pupil looks like a black dot but is actually an opening, and it changes size depending on how much light is available. When it is very dark, the pupil becomes larger to allow as much light as possible into the eye. Once light passes through the pupil, it hits the lens. The human lens is just like a glass lens. It causes light to slow down. The lens is attached to the rest of the eye with muscles above and below it. When you focus on an object, these muscles move to either squash or pull on the lens. This causes the light rays to meet at different points.

 

In a perfectly functioning human eye, the light rays meet on the retina, forming an exact image of the object seen. The retina is connected to many nerve cells, which translate the image to the brain, allowing us to understand what we see! This very elaborate process takes only milliseconds.

A diagram of the human eye. The thin green layer is the retina, and the white
A diagram of the human eye. The thin green layer is the retina, and the white translucent bulge represents the lens, Credit: Wikimedia/Pereru

Why do people need glasses?

With how many dedicated parts there are in an eye, is it any surprise that eyes do not work quite right in 79% of people?

 

Glasses are meant to correct for problems in the lens. When the lens is not formed correctly, it can cause light rays to converge either too close to or too far from the retina. The brain cannot translate the image properly, causing it to be blurry and out-of-focus. The farther the image forms from the lens, the higher the “power” of the prescription needed.

 

Diverging lenses cause the rays to be moved farther apart when they enter the eye. When the image forms too far in front of the lens, a diverging lens causes it to form farther behind, exactly on the retina. A diverging lens corrects for nearsightedness. A converging lens is used when the image is formed too far behind the lens, for people who are farsighted.

 

A thin piece of glass can easily fix an error in one of the most carefully arranged organs. These lenses have evolved from a plain cut emerald, too expensive for anyone but an emperor, to something anyone can pick up at a department store.

Glossary

Absorb: The process by which light is stopped by an object

 

Bifocals: A type of glasses that work for both nearsighted and farsighted people

 

Converging lens: A type of lens that causes light rays to meet

 

Concave lens: A type of lens where the glass bends inward

 

Convex lens: A type of lens where the glass bends outward

 

Diverging lens: A type of lens that causes light rays to move apart

 

Laser corrective surgery: a surgery that uses a laser to reshape the cornea (the clear covering over the front of) of the eye and so correct nearsightedness, farsightedness, or astigmatism

 

Lens: A thin strip of glass which causes light to refract

 

Pupil: An aperture in the eye through which light can pass

 

Reflected: The movement of light back to the direction from which it came

 

Refracted: The change of the path of light due to a change in its speed

 

Retina: A thin layer on the inside of the eye that receives light and translates it into images that are sent to the brain to be understood

Flesch Kincaid Grade Level: 6.9

 

Flesch Kincaid Reading Ease: 68.6

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Author

  • Yamini Srikanth

    Yamini's (he/they) interests lie in environmental education, science communication and trying to build a better world. When not languishing in front of his laptop, they can be found outside, poking at any insect, bird or plant. They love making science accessible, especially to those who aren't encouraged to pursue it. Yamini hopes that the young women who read Smore love learning from their articles and get just a little bit more excited about science!