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?
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.
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).