Circular surface-tension standing waves can be clearly demonstrated as follows: Fill a paper or styrofoam cup to the brim and then add a little more, so that the water bulges above the top of the cup. Tap the cup gently. Watch! To see the waves easily look at the reflection of the sky. Alternatively, use a small bright light several feet above the surface, and look at the pattern on the bottom caused by the lenslike effect of the waves. To see that surface tension is involved, try adding a little detergent to the water.
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