Glass, water, and the eye are three different transparent mediums that change the direction of light, a phenomenon called refraction. Refraction in the eye helps us see by bending light into an accurately focused point on a layer of light sensitive-tissue called the retina.

But what if you have refractive errors — a term to describe when your eye shape bends light too much or too little, or for when your eyeball is too long or too short? Educators Andrew Bastawrous and Clare Gilbert explain how eyes work and how glasses help us see for TED Ed.

Related reading: Myopia, hyperopia, astigmatism, and presbyopia.

Next: How do your eyes perceive color? What are those floaty things in your eye? Can you trust your eyes? Plus: Two close up videos of the human eye.

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