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Question:
I have noticed a phenomenon that I have never seen reference to: When I allow water from the kitchen sink to splash on a flattened metal spatula, the water flows away, of course, in a flattened, symmetrical pattern. But if I do this under a fluorescent light, I can detect a faint pattern of concentric rings around the stream of water, on the surface of the metal. I take this to be some sort of strobe effect, but I am puzzled as to what is moving in the water at a regular rate, and further by my assumption (up to now) that the nature of fluorescent lamps was such that they did not actually flicker as some other types of lights can do, at a 60 cycle rate, maybe because the gases inside were s somehow more consistently excited. Whatever. Anyone notice this before and want to enlighten me? Thanks.



Replies:
First of all, fluorescent lights flicker even more than incandescent lights, because incandescent lights take a longer time to dim when the voltage falls (between the AC cycles) because they must cool down. To prove this look at a 100W light bulb that is off. Close your eyes, turn on the bulb, then turn off the bulb and *immediately* open your eyes. Having prevented being dazzled by the full light of the bulb you will easily see the light bulb grow dim. You will not see this with a fluorescent bulb. Second of all, what might be moving regularly in your water flow is the pattern of ripples on the surface of the water flow, and of course if you did have a strobe going you would see the pattern standing still (Harold Edgerton had many famous pictures of this sort of thing.) Third, it is possible a standing wave pattern might occur in your flow, in which case the strobe is not necessary. These ripples may cast shadows beneath them, and these "shadows" need not be darker than the surroundings, but can easily be brighter. What you would be seeing would be the light being alternately focussed and defocused by the curved surface of the water. The shadows on a pool bottom, for example, that are cast by the ripples on the surface are both darker *and* brighter than the general level of illumination.

christopher grayce



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