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Anderson Video - Glasses

Professor Anderson
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<font color="#ffffff">So when you go get your prescription at the eye doctor.</font> <font color="#ffffff">It will say on your prescription for your contacts, or your glasses.</font> <font color="#ffffff">It's basically the same idea.</font> <font color="#ffffff">You're going to get a prescription that says plus 2.5 or plus 3.0 or plus 3.5 or negative.</font> <font color="#ffffff">What does that number mean?</font> <font color="#ffffff">What is the lens power?</font> <font color="#ffffff">Power, or more specifically refractive power, is 1 over f.</font> <font color="#ffffff">Okay, it's just 1 over the focal length of the lens.</font> <font color="#ffffff">So this is measured in something called diopters.</font> <font color="#ffffff">Diopters is, of course, just 1 over meters.</font> <font color="#ffffff">All right, so if you have a plus 1 lens.</font> <font color="#ffffff">If your power equals plus 1, then that means the focal length is 1 over that.</font> <font color="#ffffff">It's 1 meter.</font> <font color="#ffffff">But if you have a power of 2, focal length is one over that which is a half a meter.</font> <font color="#ffffff">Okay, so my glasses here.</font> <font color="#ffffff">The left eye is a plus 3 and the right eye is a plus 2.5.</font> <font color="#ffffff">And that means that the focal length here is about a third of a meter on this side.</font> <font color="#ffffff">Okay, 30 centimeters.</font> <font color="#ffffff">If you have negative numbers for your prescription, then you have diverging lenses.</font> <font color="#ffffff">So if your power equals negative 2, then your focal length is negative 0.5 meters.</font> <font color="#ffffff">Okay, and so when you look at your prescription, you can tell right away.</font> <font color="#ffffff">Do I have eyes that need a big converging lens?</font> <font color="#ffffff">A lot of power.</font> <font color="#ffffff">Or something that's a little more relaxed, a little more loose.</font> <font color="#ffffff">Okay, so let's think about power in terms of your eyeball.</font> <font color="#ffffff">Okay, what we said was the power is going to be 1 over the focal length.</font> <font color="#ffffff">And now we're worried about the focal length of your eye.</font> <font color="#ffffff">And we talked about the focal length of your eye last time.</font> <font color="#ffffff">It has to be basically the size of your eye.</font> <font color="#ffffff">Which is about 25 millimeters. Okay?</font> <font color="#ffffff">So this is 1 over 25 times 10 to the minus 3 meters.</font> <font color="#ffffff">Which is 40 diopters.</font> <font color="#ffffff">Okay, that cornea is very curved.</font> <font color="#ffffff">It's a very powerful lens.</font> <font color="#ffffff">These are sort of typical numbers for your glasses, right?</font> <font color="#ffffff">So when I think about the glasses that I put on to read and I'm gonna add a power of 2.5,</font> <font color="#ffffff">That's going on top of 40 diopters already.</font> <font color="#ffffff">So it's a small fractional change</font> <font color="#ffffff">And the way you can tell this is if I just look at the glasses, edge-on, they're not very curved.</font> <font color="#ffffff">Okay, they're a little bit curved, but not very.</font> <font color="#ffffff">And if I look at my eyeball, edge-on, it is really very curved, right?</font> <font color="#ffffff">It looks like a marble.</font> <font color="#ffffff">The more it's curved, the shorter the focal length, the bigger the lens power.</font>