Edited by author 12-02-2005 02:28 AM
My apologies for disagreeing with the person in the audience who said strobes vary in their duration. From what I read on the web they do to accommodate different distances, in one case from 1/1,000 sec to 1/50,000. I assume they stay on longer for objects that are further away, as the surface area to illuminate increases with square of the distance.

Which made me thing of another science fair project. Measure the duration of strobe flashes with a rotating luminescent wheel. Put the disk in a box, cut a hole for a lens to focus the strobe to a point on the surface of the disk, and spin the disk at 1200 rpm. The strobe will leave a curved streak on the wheel, whose length (angle subtended) is proportional to strobe duration. Why 1200 rpm? Because it's fast so the streak will spread when the strobe is on, and it's easy to calculate. 1200 rpm = 200 rev per sec. A 1 millisecond strobe will make an arc 1/5 of a rotation. A 1/50,000 sec strobe will make a 1/250th of a rotation, less than a degree (250/360).

Well thank you for the question. I have two students I'm going to see tomorrow who need a project. Guess what I'm going to tell them about? I bet they are going to have fun trying to spin something that fast. I'm thinking small and view it with a magnifying glass. And wear those safety glasses!!!