How do glasses work?

Glasses have been humanity's go-to solution for vision correction ever since we discovered how to grind glass. But how do glasses work? Like, the actual physics of it? The short answer is that glasses work by bending light before it reaches the eye. There's a lot more to it than just this, of course, and knowing how your glasses are supposed to work can help you find the right set for you.

Understanding vision and refractive errors

Vision happens as a sequence of events, starting with light bouncing off of a surface and streaming toward your eye. As the light particles enter through the cornea and lens of your eye, the beam gets bent. This is called refraction. If things are going well, this refraction should focus a sharp image on the light-sensitive back of your eye, which is called the retina. Special cells here catch the light and send signals back up the optic nerve to your brain for image processing. 

If the light falling on your retina has a focal point too far forward or back, your light-sensitive cells will only get a blurry image. This can happen because your lens or cornea is a little out of shape. Up to a point, your eyes will automatically correct for this by flexing or pulling your eyes into a slightly different shape, but there's only so much they can do. When your eyes aren't focusing properly, the result can be three common vision problems.

Nearsightedness

Nearsighted people's eyes tend to focus images too far in front of the retina. Also called myopia, this is one of the most common reasons people need glasses. If you're nearsighted, you can probably see things clearly up close, but objects far away tend to be blurry or out of focus.

Farsightedness

Farsightedness, also called hyperopia, is the opposite of myopia. If you have this, you may have trouble seeing up close, but your distance vision can still be sharp. This condition is caused by having images focus too far back, with a theoretical focal point that lies behind the retina.

Astigmatism

People with astigmatism have eyes that form two or more images, which may be in front of or behind the retina. They might also have images forming at the right distance, but set to the side, away from the fovea, which is the part of the retina where most of the light-sensitive cells are clustered.

The science of refraction

Refraction is the bending of light as it enters a medium. When light passes through the lenses of your eyes, it changes speed a little bit, which makes it bend. The degree to which the light bends is called the refractive index, and it's a result of two things: the amount of slowdown and the angle light enters the lens. By altering these factors, lens grinders can set the refractive index, which becomes your prescription.

How glasses correct vision

Glasses work by bending light before it reaches your eyes, effectively canceling out the distorting effect of your natural lenses. When you get an eye exam, they're testing your vision to figure out the refractive index of your natural lenses. Then, they do a calculation to figure out how light would have to be bent before it gets to your eyes to compensate for this effect. That new refractive index is your prescription for glasses. Ta-da!

Prescription and lens power

The refractive index your eye care professional says you need for vision correction is expressed in a unit called diopters. The higher the number of diopters your prescription calls for, the more vision correction you're getting. Uncorrected vision is a 0, while progressively stronger prescriptions run positive and negative away from that point.

Types of corrective lenses

There are several different types of corrective lenses. Depending on what kind of refractive index you need, you can go with concave, convex, or cylindrical lenses.

Concave lenses

Concave lenses treat nearsightedness. These lenses are thinnest in the center (think of how a cave looks, which is how to remember the difference between this and convex lenses). The numerical prescription in diopters is marked with a minus (-) symbol. The lenses in concave lenses tend to spread light out and move the focal point backward, allowing images to sharpen on the retina.

Convex lenses

Convex lenses do the opposite of concave lenses. Instead of pulling the light apart to make it focus farther back, these lenses pull it inward to focus farther forward, correcting farsightedness. The numerical prescription for convex lenses is always marked in diopters as a plus (+) symbol.

Cylindrical lenses

Cylindrical lenses are used to treat astigmatism. Because this is a more complex seeing problem than the other two, the lenses you need to treat astigmatism are more complex and may have to be custom designed to match the distortions caused by irregularly shaped corneas or lenses.

Factors affecting visual clarity

There's more that goes into the performance of your corrective lenses than just their refractive index. Other factors include:

• Lens material. Lenses tend to be made from glass or plastic, which affects durability, longevity, and the difficulty of getting the right grind. Plastic lenses also tend to be lighter than glass and may be a better choice for all-day wear.
• Coatings. Several coatings are available that can improve the performance of your lenses. Popular choices lend your lenses scratch resistance, UV protection, and glare reduction.
• Frame fit. How your frames fit affects how they perform for you. Be sure you get a set of frames that fits your face and doesn't move around too much while you go about your day. Because the refractive index of prescription glasses can be affected by the angle light moves through them, it's important to have frames that hold them in the right position.

How do glasses work, and what does it all mean for you?

Glasses are marvels of Medieval technology, which have developed into modern wonders. You have to have the right glasses to see well though, so schedule an eye exam to get your prescription now.

FAQs

How often should I get my glasses prescription checked?

Most eyeglass prescriptions expire after 1 or 2 years, so it's a good idea to get an eye exam around then to make sure you don't need a change. You should also get checked if your corrected vision starts to blur because your old prescription might not be working for you like it used to.

Can wearing the wrong prescription glasses harm my eyes?

If you're an adult, wearing the wrong prescription probably won't do anything worse than strain your eyes. For children, the wrong glasses can actually accelerate the development of vision problems, so it's really important to get them the right glasses and have them periodically adjusted to keep up with changing vision.

Are there specific lens coatings that enhance the performance of glasses? 

Modern lenses are more than just glass. Today, many people opt for lens coatings that add extra features to make glasses even better for people who need vision correction. Specific coatings include:

• Anti-reflective. This cuts down on glare and reflections and may improve performance overall.
• UV-protected. These coatings reduce the flow of ultraviolet light through the lenses and may reduce damage caused by sunlight on the corneas.
• Scratch resistance. These coatings are tough and reduce the risk of scratches and small chips that mar the surface of lenses.
• Blue light. This coating helps cut down the fatiguing light from computer monitors and other screens.