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I realised that when a normal ceramic cup is placed on a table, and the table is knocked repeatedly, there seems to be a single focal point where there are circular waves on the inside of the cup through the water as long as there is light shining inside the cup, not on the water. It is hard to explain, but no matter how you tilt your head, the circles form on the same point on the inner surface of the cup. Why is this so?

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