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 (See Figure
1).
If the stroboscope flashes twice as fast you see two arrows (See Figure 2) as the
disc has only rotated half a turn between flashes. If it flashes three times as fast then you see
three arrows (See Figure 3) and four arrows if it is rotating four times as fast (See Figure
4).
Of course 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.
If the arrow in Figure 1 drifts anticlockwise it means that it has gone
round a little less than one revolution between flashes. This means that the disc is rotating a
little slower than the strobe is flashing. A clockwise rotation means that it is going round a
little faster than the strobe is flashing.
Stroboscopes are used for checking the timing
of car engines, looking at moving machine parts, and checking the speed on record player
decks.
