How to tell if glasses are nearsighted or farsighted

by Stephanie Chasteen on August 21, 2008

A neat observation from one of the staff physicists at the Exploratorium:

Here is a little game to play with farsighted and nearsighted glasses. Ask all your students who wear glasses to put them on and stand up. Walk up to each of them, look into their eyes and you will be able to tell them if they are nearsighted or farsighted.

If they are farsighted (and therefore have convex lenses) you will see the contour of their cheeks move OUT when viewed through their glasses. If they are nearsighted (and therefore have concave lenses) you will see the contour of their cheeks move IN when viewed through their glasses. This is a nice opportunity for a ray diagram or two! Astigmatism, graded lenses and bifocals can make this more difficult, but it is fun to try. The stronger the prescription the better. Holding far and nearsighted glasses up to colored lights or shadows also produces discriminating effects.

This could be a great “nature of science” activity! Tell them you have mystical powers and can see the shape of their retina (or some such garbage) just by looking deeply into their eyes. (Of course, it won’t work with any students who wear contacts! Why not? Can they guess how you do it?)

{ 4 comments… read them below or add one }

Nonnormalizable August 22, 2008 at 8:33 am

I like the concept, but will they really be surprised by the teacher’s ‘ability’? Maybe I’ve giving the average student too much credit, but as someone with and from a family having very strong prescriptions (for nearsightedness), this is a super obvious and ubiquitous effect. Pictures of us -8 diopter folks often look freakish because an image of the background ends up appearing effectively inside our face. Eh, probably you’re right and most wouldn’t get it right away.

sciencegeekgirl August 22, 2008 at 9:51 am

I’ve been surprised by the kinds of “magic tricks” that teachers have reported fooling their kids with, at least at the middle school level. Now, I’m not a teacher, so I certainly can’t vouch for it at all, I’m just throwing it out as a possible fodder for discussion!

Wholesale Reading Glasses September 5, 2008 at 8:59 am

That trick is really cool, I’ll try it the next time I go to a bar and will win loads 🙂

On a side note, some really cool stuff on here and your really cute to boot.



Awesome Jordan June 17, 2012 at 5:58 am

i might get glasses for nearsightedness

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