Science Questions

Why can't I see my watch when wearing sunglasses?

Sun, 25th Nov 2007

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Ryan, Michigan asked:

While wearing sunglasses, Iíve noticed that I can no longer see the image on the face of my digital watch. Why?


Ryan is probably wearing whatís called polarised sunglasses.  What they do is they only let one polarisation of light through.  Light is a wave and can be thought of as a bit like a piece of string.  So if I had a piece of string between me and you there I could probably loosen it by wobbling it up and down and that would make the waves move vertically.  I would call that vertically-polarised light.  Or I can make waves by wobbling it horizontally and that would be called horizontally polarised light.

Light can be either of these, or any other polarisation in between.  In fact, light coming from a normal light bulb or a fluorescent tube or the sun has all of them mixed together. Every possible polarisation.

Then you get things called polaroids which your sunglasses are. These will only let one polarisation through. If you have them horizontally it will only let horizontal through or vertical will only let vertical through.  If youíve got two of them, if theyíre both horizontal light can get through it because light can get through the first one and the second one. If you rotate the second one to vertical nothing will get through because the first one stops horizontal and the second one stops vertical.

LCD's use this because they have polarisers on them. Light comes though from underneath and gets polarised so only the vertical is coming through.  Then you have something called Liquid Crystals which can twist the polarisation of light by 90 degrees: they are set up so light can get through normally.  But if you then apply an electric field to it, the crystals stop twisting the polarisation so no light will get through and itíll go black.  This means the light coming out of the LCD (Liquid Crystal Display) is polarised in only one polarisation so if your sunglasses are orientated in the right way it can block it and itíll go blank.


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