How Do Glasses Work?

How do glasses work?

If you wear glasses, you know that they make your vision clear and sharp when you put them on. But how does this work? In this blog, we’ll discuss the science behind corrective lenses and how they change the way we see.

Refractive Error

The term “Refractive Error” refers to the eye’s inability to focus images correctly. This causes blurry vision and is usually treated with corrective lenses. The 3 common types of refractive errors are Myopia, Hyperopia, and Astigmatism.


Myopia, or nearsightedness, occurs when light comes to a focus in front of the retina. This occurs either because the eye is too long, or because the cornea is too curved. People who have myopia can usually see images close up, but cannot see at a distance. 

Graphic of an eye with myopia

Myopia is corrected with concave or minus lenses. These lenses increase the focal length of light rays by divergence, allowing them to stretch and reach the back of the eye, where they will come into focus. 

This image shows light rays diverging to increase their focal length.

Concave lens diverging light rays


Hyperopia, or farsightedness, occurs when light comes to a focus behind the retina. People who have hyperopia can usually see images better at a distance, but cannot see up close. If hyperopia is severe, they may not see well at either distance.

Hyperopia is corrected with convex or plus lenses. These decrease the focal length of light rays through convergence, allowing them to come to a focus on the retina.

This image shows light rays converging to decrease focal length.

Convex lens converging light rays


Astigmatism occurs when light rays reach multiple points on the eye. There are several types of astigmatism, depending on where the light rays are focused. 

Graphic of an eye with astigmatism

Glasses correct astigmatism with toric lenses. These lenses have different powers aligned at the place on the eye where light rays travel through. This allows light to be focused on the correct part of the retina. 

How does my eye doctor know what lenses I will need?

The power of your lenses is determined by your refractive error. When you see an eye doctor for a new glasses prescription, they will perform a refraction. This is the test that determines how strong your lenses need to be in order for you to see your best. This is a subjective test that relies on your input. Your doctor will use a piece of equipment called a phoropter to hold lenses with different strengths in front of your eye. You will then tell the doctor which lenses make your vision most clear. Everyone needs a different prescription for glasses, because everyone’s eyes are different. 

This image shows a patient behind a phoropter during a refraction.

Person sitting behind a phoropter during a refraction

Thank you for taking the time to read our blog. To learn more about your eyes and how they work, check out the blog section of our website at To schedule an eye exam, call or text (907) 328-2920.


“File:Concave lens.jpg” by User Fir0002 on en.wikipedia is licensed under CC BY-SA 3.0

“File:Large convex lens.jpg” by User Fir0002 on en.wikipedia is licensed under CC BY-SA 3.0

“eye exam” by Robert Couse-Baker is licensed under CC BY 2.0

Written By: Gina Stafford COA, LDO, ABOC

Posted in: Uncategorized

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