why does wearing other people’s prescription glasses make your head hurt?

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i used to wear my nan’s glasses for fun a lot as a kid

In: Physics
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You’re basically seeing through someone else’s eyes, and doing that causes a lot of strain on your own. The more strain on your eyes, bam, headache.

So start with this: Your brain just wants your eyes to see as best as it can.

You’re used to the way light enters your pupils and projects onto the back of your eyeball. You have a special curvature of the eye that warps and bends the light, you have your own special arrangement of cells that fire when exposed to light, this is an arrangement that you’ve been adjusting to your whole life, starting since the moment you opened your eyes.

Small changes over long periods of time don’t really hurt much. I used to have perfect vision but as I got older the curvature of the lenses of my eyes and the way they fit into my eye socket changed gradually, so I didn’t notice the change until I failed the vision test at school.

Glasses are essentially prosthetic eye lenses: your eye is curved, not only because it can roll around to look more than straight ahead, but also because that bends light in a way that you can see a sharper image and a wider field of view. Glasses correct the light’s contour in a way that you can see better.

Now imagine me going from the perfect eyesight of my childhood straight to the warped, blurry stuff I see today. Pretty jarring, right?

Glasses take it a step further: the lenses bend light a lot more drastically than the lenses in our eyes, especially glasses with thicker lenses. So if you put on a prescription that doesn’t fit you, you’re experiencing an image made of light bent in a way you’re not used to at all. So what your brain, which just wants to see well, decided to do, is adjust everything to get what it’s used to. If tenses all the muscles around your eyes, squishing the eyeball slightly to warp its lens, doing anything it can to get what it had before, but it just can’t get there.

This strain in the muscles on your face combined with the slight dizziness from the warped image you get probably contributes to the headache you get from putting on someone else’s glasses. You don’t get this from the sudden change of putting on the right glasses, because your brain is content with the sharp, clear image it’s getting, it doesn’t need to strain anything. And you also don’t get this from the sudden change of taking them off, because nobody wears their glasses 24/7, so they’re still accustomed to how their eyes naturally see the world.