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This glass seems to have no ripples in it when viewed from the side, but lots when viewed from above (see photos). Why is this?
鈥� The 鈥渞ipples鈥� are caused by multiple reflections as light from the base of the glass bounces between the inner and outer surface of the glass on its way up. At each reflection a little light escapes and enters the eye, producing the observed 鈥渞ing鈥� at that height. If you look down the outside of the glass from above, you will see ripples once again.
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Keith Thompson, Bendigo, Victoria, Australia
鈥� The effect is caused by internal reflection, just as happens inside an optical fibre. To see a neat demonstration of this, cover the sides and one end of a glass tube with black plastic tape, then punch a hole about 1 millimetre across in the sealed end. Now look down the small hole and you will see concentric rings caused by internal reflections. If you look through the unsealed end you get some pretty interesting views too.
I have used this effect as a little test when interviewing prospective physics students at Oxford.
Mike Glazer, Clarendon Laboratory, University of Oxford, UK