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Author Topic: Why do LED lights create patterns on moving water?  (Read 2336 times)

Offline kiridaust

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I first noticed this phenomenon looking at the water flowing along the bottom of the sink where the stream from the tap hits. When I shone an LED light onto the bottom of the sink, concentric circles appeared, with a centre point where the flow from the tap hit (see picture below). The distance between the circles decreased as the distance from the centre increased. I later noticed a phenomenon that I assume works on the same principle--an LED light creates dashes on the water flowing up and falling back down from a fountain (or flowing from a kitchen tap).

The only potential explanation I currently have for these patterns is that the flicker of LEDs creates an interference pattern (I've found out that LEDs make a single wave for a particular colour and guessed that the on-off could produce different waves that could interfere) which can only be seen on moving water. Does that make sense? I don't have measurements yet, but I tried changing the water flow--and I hypothesized that the pattern shouldn't change--but it seemed that the distance between the concentric circles was smaller with lower flow of water (thus decreasing the speed it flows along the sink)--I'll have to check on this more closely--I'm having trouble getting photos that I can measure. I'm also not sure why an interference pattern would create concentric circles from the tap? None of the hypotheses I come up with seem to explain it fully and I haven't found anything useful on the web. If anyone has any ideas, I would be fascinated. Thanks! 


 

Offline alancalverd

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Re: Why do LED lights create patterns on moving water?
« Reply #1 on: 18/09/2014 06:33:11 »
Many LED lamps flicker, either at mains frequency or because they are driven by a "chopper" circuit from a battery to regulate the current through the diode, so my guess is that you are seeing a stroboscopic effect and the pattern is actually surface waves on the water. However if the flow rate is very small and the water is barely covering the sink surface, you can also get an interference pattern (see "Newton's Rings") between the light reflected from the top and bottom surfaces of the water - though I think this less likely as you seem to be using a "bright white" LED source, which is not monochromatic. What is the scale of the photograph?
 

Offline kiridaust

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Re: Why do LED lights create patterns on moving water?
« Reply #2 on: 18/09/2014 20:40:42 »
I don't think that a stroboscopic effect showing surface waves really explains everything. If you look at the pictures below of a fountain above an LED light, you can see that evenly spaced dashes are created on the flowing water--they don't change position, so how how could that be anything other than an interference pattern? You can see on one picture below that the bands seem to run across the fountain, no matter what stream of water is showing it.
 

Offline chiralSPO

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Re: Why do LED lights create patterns on moving water?
« Reply #3 on: 18/09/2014 21:02:09 »
Actually, I think the fountain pictures suggest stroboscopic effect rather than interference. Like the waves produced on the surface of the water in your sink, the fountain is producing droplets of water that fall at a certain frequency. If the frequency is very close to that of the strobe (or some harmonic of it) then you see stationary droplets of water (the further the frequencies are, the faster the droplets will appear to fall.) It's just like car wheels that appear to be stopped or moving backwards on a film of a car in motion.

If it were interference, you would probably also see a dramatic rainbow effect because the different wavelengths of light would diverge in the image.
 

Offline evan_au

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Re: Why do LED lights create patterns on moving water?
« Reply #4 on: 18/09/2014 21:53:30 »
I think the clue in the fountain photo is that the length of the "dashes" increases as the droplet falls to the ground.
  • The LED power supply is pulsing at a fairly constant frequency and duty cycle*
  • The droplet is illuminated by flashes of constant duration.
  • The droplet is accelerating under gravity as it falls.
  • So the droplet travels further during the flash.
  • Hence the dahes get longer as the droplet falls further 
  • So I think it is a stroboscopic effect, but one which is independent to the frequency of droplet generation. 

*Duty cycle is the ratio of the ON:OFF times of the flahes

Quote from: chiralSPO
the fountain is producing droplets of water that fall at a certain frequency
It is possible for dripping water to exhibit periodic-like behaviour when it is dripping slowly (and then break into twice the frequency as it randomly starts double-dripping).

But an unmoving spray head releasing thousands of drops per second is designed to operate in full chaotic mode, where there should be no discernable pattern in the release of water droplets (ie it effectively becomes random). In this case, the flashing nature will be seen most clearly further away from the spray head, where there are single drops falling.

Of course, you could make a spray head with a little whizzy thing inside that released droplets at a specific frequency, and this would show stroboscopic effects at certain speeds/water pressure and not others. In this case, the stroboscopic effect would be strongest closest to the spray head, and effectively disappear further away. This is not the case in the fountain photo.

To resolve this question, have a close look at the dashes - to my eye they don't seem to be aligned - but to tell, you would need to get a photo taken horizontally.
« Last Edit: 18/09/2014 22:09:21 by evan_au »
 

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Re: Why do LED lights create patterns on moving water?
« Reply #4 on: 18/09/2014 21:53:30 »

 

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