Measuring Frequency with a Stroboscope

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We are going to see how to use an electronic stroboscope in order to measure frequency. This is an electronic stroboscope, or just called strobe for short. Its simply a flashing light; it flashes at a regular rate and that rate is adjustable. Right now it's flashing at 3600 flashes per minute, and I am decreasing the rate right now. I can either increase or decrease it; I can do it in big steps or in small steps.

Now you are probably familiar with strobe lights, for example the strobe effect in the video clip or strobe lights that are used in parties. But actually the strobe light was invented back in the 1930's, and it was used originally to investigate the motion of motors, because the motor was moving too fast to be seen with the naked eye, but with a strobe light, you could make it appear as if it were stopped. I'm going to show you how that works.

We are going to be using for a motor this fan motor, without the blades on it. Instead of the blades, there is this cardboard disc which has a white line painted on it. I want you to think of this as a clock. I'll turn it on. It's a high-speed clock, its moving too fast for you to see the hand, but when lighted with a stroboscope, you will be able to see the hand. Let's take a look at that now. So you can see the hand, actually you can see multiple images of the hand.

What I want to do is make it look like there is only one image of the hand and that it is moving as slow as possible. You will see why we want that effect in a moment. I'm decreasing the frequency now, in big steps. I'm now down to 3200 flashes per minute, now 3100 flashes per minute. Now you notice that the line appears to go one way and then stop and go the other way. First of all, I can adjust this and make it look like the line is stopped. It's pretty nearly stopped now. I'll leave it like that for the moment. Its still moving of course. If it were completely stopped, or appeared to be stopped, then what that would mean is every time this light would flash once, the line or disc went around once. So the first time the light flashes, you see the line in a particular position. The next time it flashes, the disc has gone around but the line appears to be in the exact same position, and that's the only time you see so it appears to have stopped. If it appears to be going slowly forward like this, that means that the light is flashing a little too slowly. So let's suppose that the line starts right here and between flashes. It goes around once and a little bit more, since this light is flashing a little too slowly. And the next time it goes around once, back to its original place, and a little bit more. So it appears to be slowly moving forward, if this light is flashing too slowly. Likewise if the light is flashing too fast, the line appears to be moving backwards. Now you have probably seen an effect like that in movies, for example stagecoach wheels appear to be going backwards in a video clip. This is because the video clip, itself, is like a strobe light. It has 30 frames per second, so its like 30 flashes of light per second, which are blended together so you don't notice the flashing. But on something like a stagecoach wheel that is moving you see the stroboscopic effect.

Now I said before that when we had the line appearing to be stopped like this, this light is flashing once every time the disc goes around once. Actually there are other possibilities. Its possible that the disc is going around twice every flash. If you were going around twice, there's once, twice. It would look like its in the same place every time. It could be going three time or four times between flashes. We need a way to determine if its doing that. Let's suppose that, right now, it is going around exactly once between flashes. If I then doubled the frequency of the light, then what would happen, if the line started here, when the light flashed again it would get half as far, because the light has half as much time between flashes. And for the next flash, it would be here, then here. So we would appear to get two lines, two images of the hand if we did that. So let's try that and see if it actually happens. If it does happen, then this frequency is exactly what we said it was in the beginning, this is going around once for each flash of the light. I'm hitting a button one here called times 2, so the frequency is twice as much, you can see that two lines appear, so that means that the frequency we started with was the correct one, the one that we wanted. I can hit divide by two to get back to where I was. And that frequency is 3130 flashes per minute. So that's how we use a stroboscope in order to measure frequency. This can be used to measure the frequency of many things that move at a regular rate, like rotating objects or vibrating strings. Another practical application is timing lights for automobiles.