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Kitchen Sink Strobe
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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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Update: June 2012
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