The other day when I was cycling back from work in the rain I noticed an effect from the light on my bike. A single raindrop, as it passed the light, appeared as multiple dots or dashed that followed the expected trajectory of the raindrop. I haven’t thought about it before but I would have thought it would appear as a streak or just single flecks. Is it something to do with modern led lights?

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    $\begingroup$ Well spotted! Noticing things is good. $\endgroup$ – DrC Dec 23 '19 at 16:32
  • $\begingroup$ Hi there! Welcome to this very good site, as you will notice in time. In fact, you've already answered this question by yourself, though you were not sure which is why asked it. That's why downvoted. $\endgroup$ – Deschele Schilder Dec 24 '19 at 12:39
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    $\begingroup$ You will see essentially the same effect in the light from mercury or sodium street lamps. Sweep your gaze past street lamps and you'll often see a string of bright dots. It's because those lamps flash at a rate determined by the 60 Hz AC voltage that's driving them. $\endgroup$ – S. McGrew Dec 24 '19 at 15:40


LED lights are often fed from a circuit that causes them to flash so quickly they look continuous.

What you are seeing is the stroboscopic effect.

Cheaper lights use a resistor to control the current through the diode, but this wastes energy. The pulsing circuit increases battery life.

  • $\begingroup$ In Are LEDs in modern streetlights usually pulsed? If so, roughly what frequency? answers claim that for overhead streetlight applications, they use a DC supply. I'd naively thought that all LEDs were PWM regulated. $\endgroup$ – uhoh Dec 24 '19 at 21:50
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    $\begingroup$ A constant current DC supply can be used. The supply is usually generated by pulses for efficiency purposes but smoothed subsequently to prevent flicker caused by slightly different pulse frequencies in nearby street lamps. $\endgroup$ – DrC Dec 25 '19 at 13:09

Brightness of LED sources is controlled with PWM (Pulse Width Modulation) -- the LED flashes (time_on/time_of - duty cycle). Due this 'effect' you see rain drop multiple times with gaps. Human eye is capable distinguish movie 'frames' with frequency below 24Hz (nowadays a good TV provides 120Hz refresh rate).

  • $\begingroup$ Mmm. You can often see the flashing if you look off to one side of the LED, and then look directly to the other side; the flashing makes the LED look briefly like a dotted line across your vision (instead of the solid line it would be if steady) — turning the temporal cycle into a spatial one, in the same way the raindrop did. (This works best in the dark, and can take a bit of practice.) $\endgroup$ – gidds Dec 24 '19 at 11:56
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    $\begingroup$ @gidds First, I would like to wish you a very happy and wonderful Christmas. The led-light, in this case, is as well temporarily as spatial cycled and it depends on the frequency if it's visible or not. But this is just a comment, made to make it somewhat clearer. $\endgroup$ – Deschele Schilder Dec 24 '19 at 13:04
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    $\begingroup$ I made solar powered traffic signals and we used this technique. On cloudy days or at night, we expanded the dark space between pulses. Because your pupils dilate in lower light, the traffic signals look very bright, even though the LEDs are only illuminated 10% of the time at night or 50% under clouds. During the day, the LEDs were illuminated nearly 100% of the time, with very short gaps between the pulses. The number of pulses per second doesn't change; only the space between the pulses. $\endgroup$ – foolishmuse Dec 24 '19 at 16:08
  • $\begingroup$ see this comment $\endgroup$ – uhoh Dec 24 '19 at 21:50

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