Strobe Fountain
The Science Centre Singapore has an exhibit in which falling water droplets can be apparently suspended in mid air and made to slowly move up or down by turning a knob one way or the other. It's a popular exhibit, but many people find it difficult to understand the explanation provided. Here's an attempt to assist in the explanation with an interactive visualisation that allows you to slow down time to see what happens.
The Strobe Fountain exhibit is based on the stroboscopic effect. Under normal light, a falling droplet is visible all along its path, and we perceive it as a falling droplet. Under a stroboscopic light – which is darkness most of the time with brief flashes of light at regular intervals – the droplet is falling in darkness and illuminated every few milliseconds.
If it was a single droplet, we would see it jumping trough a sequence of illuminated positions as it falls. The trick here is that we don't just have one droplet but a continuous series of droplets falling one after the other, with more or less equal distances between them (at any given position in the path). So while the first droplet has jumped to the next position, the second droplet has replaced the first one where it was before – if the regular timing of the flashes matches the regular sequence of droplets, that is. And because the droplets all look more or less the same, we perceive the succession of droplets appearing in the same spot as one droplet suspended in that spot.
If we now slightly change the time interval between flashes (ie the strobe frequency), we can make those apparently suspended droplets move slowly up or down. If the interval is shortened, the droplets don't quite reach the positions of their predecessors by the time the next flash occurs, so we see the positions of the suspended droplets slowly moving upwards.
In this slow-motion version of the Strobe Fountain, you can adjust the strobe interval (the time interval between flashes) just like in the real exhibit. (Try doubling or halving the interval and see what happens.) But unlike with the real fountain, you can slow down the animation (by reducing the frame rate) to almost a standstill to really see what's going on.
