# Stroboscopes

A stroboscope is a device that makes moving objects appear to slow down or stop. It can be just a simple wooden disc with holes in that can be spun round or a flashing xenon light.

With the spinning disc we can look at moving waves, and if we look at the waves through the holes and then we spin the disc at just the right speed, the waves will look as though they are still because one crest has taken the place of the one in front of it while the disc has spun from one hole to the next. A simple disc with say 8 slots spinning at 5 revolutions per second will give you 8 x 5 = 40 glimpses of an object every second. There will be 1/40 s between one glimpse and the next.

Using the flashing light version you can watch an arrow drawn on a card and spun by an electric motor.

If the stroboscope flashes at the same speed that the disc rotates you see one arrow. If the stroboscope flashes twice as fast you see two arrows as the disc has only rotated half a turn between flashes and so on.

You would get the same result as in Figure 1 if the disc was rotating at half the speed of the strobe – the disc would then have gone round twice in between flashes.

DANGER: ALWAYS CHECK IF ANYONE IS AFFECTED BY FLASHING LIGHTS BEFORE USING A FLASHING LIGHT STROBOSCOPE.