Science Questions

Why do glasses worn backwards still work?

Sun, 8th Jan 2012

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Question

Steven Wasmer asked:

Hello Naked Scientists!

 

Thanks for an excellent show. I was wondering, if eye glasses use curved glass to bed light entering the eye to correct vision, then how is it that glasses still work if you hold them backwards (ear pieces out from the face)? Shouldn't this reverse curve make the image even more blurry?

 

Thanks

 

Steven

PA USA

Answer

Dave -  This is all to do with how a lens works in the first place.  A lens bends light based on the fact that light goes slower in glass than it does in air.  So when a light beam hits it at an angle, it slows down on the side it hits first and that tends to spin the light around a bit.  A bit like if you're driving a car and you drove one wheel into some sand,, it would spin the car around a bit.  Itís a bit like that for light.  A light ray bends around as it enters the lens, and then when it leaves, the first bit of light to leave the glass will speed up again. causing it to twist around in the opposite direction.  So, if light enters a piece of glass where both sides of it are parallel, it will bend as it goes in, but it will do the exact opposite of that bend as it goes out, meaning it will carry on in the same direction it started at.  

But if you make the two angles on the piece of glass not parallel, so if itís making a curved shape on both sides, then it will get bent differently on the two sides.  This means it will end up either pointing inwards, if it's a converging lens or going outwards if itís a diverging lens.  

If you flip the lens over, even though on the lens from apair of glasses, the two sides don't look identical, it will still be thicker on the outside than on the inside, or thinner on the outside than it is on the inside.  And so, the light will also be bent in the same direction it would be otherwise.  Although it might not produce quite as good an image, because they put different curvatures on each side to do some very cunning optimisation on the optics.

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