This question touches on the interesting subjects of photography, motion, and periodic sampling. The earliest mention of the word "stroboscope" (which matches my dictionaryís etymology of the word: strobo(s) (Greek), meaning "whirling" and scope (Greek) "to look at") appears to have been from Simon Ritter von Stampfer of Vienna who made a device from two turning wheels, one with pictures of an animated sequence, and one with shutter slits in it. An easy do-it-yourself project is to build a zoetrope
which is a very similar device.
But you may be asking about the more modern use of the word "stroboscope", meaning a device that periodically illuminates a subject with very short flashes of light. These are usually built out of tubes containing Xenon gas and a high-voltage spark is passed through the gas to make a short but very bright flash of light. The principle of periodic viewing of a moving subject is borrowed from the older motion devices and the name was borrowed; the whirling part was gone, but the periodic viewing remained. Stroboscopes are used today most commonly as timing checkers for car engines, party lights, and as apparatus for studying very fast motion, which preferably repeats. If you have a strobe light which can flash at an adjustable rate, I would recommend shining it on a spinning wheel of any sort and adjusting the rate to see what happens. A household fan makes a good test object -- try shining a stroboscope on a ceiling fan or a window fan or a table fan. You can also try illuminating a vibrating string with a strobe lamp. They look best if you have a long string with a big back-and-forth swing to it. Try a cello or a harp; guitars should be fine. They vibrate rather quickly, though, and so your stroboscope should have a high frequency (the A string of a violin is 440 Hz). You can get away with a lower frequency as long as the frequency of vibration is close to a multiple of the lower stroboscope frequency. Often fluorescent lights, which flash with the frequency of the power from the wall are enough to see stroboscopic effects on moving objects, but the effect is less clear than with a flashlamp because the flashes are not short. If you tune a string to have a frequency that is close to a multiple of 60 cycles/second, set it vibrating and look at it under fluorescent lights, it should have alternating blue and orange stripes on it (or at least thatís what it looked like to me when I did this last). If the frequencies arenít exactly a multiple but are close, then the stripes will move around within the range of space that the string is vibrating in. If the string frequency is very very close to a multiple of 60 Hz then the stripes will move slowly; if it is farther away, the stripes will oscillate more quickly.
The effect of periodic illumination of periodic motion is quite a lot of fun -- experiment with different flash rates, and for more info and exmaples read our answers to the question flickers, sampling, aliasing and persistence of vision
(republished on 07/17/06)