A stroboscopic effect is a type of effect in which a continuous motion of something is represented by a series of short or instant frame samples. For example, a fast-rotating object can be observed, either in slow-motion or even in a static mode due to this stroboscopic effect. The stroboscopic effect can be seen mostly in a fast-moving object.
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Stroboscopic Effect |
Everything we capture in the world, either through a camera or through our own eyes, is by picture by picture, which can be also said as frame by frame. By displaying these pictures continuously at a specific speed, we can make a video. This is what cameras also do. Even though we tend to capture a video through a camera, it takes it in the format of picture by picture. In movie theatres, these collected pictures are displayed on the screen, at a rate of 24 frames per second (FPS), through which we can observe it as a video. Now by keeping this frame concept we can easily understand the concept of the stroboscopic effect.
Now let’s consider the above example and rotate the arrow at a certain speed (for example 1 RPS (Rotation per Second)), which means the arrow rotates a complete rotation for 1time per second. Let’s assume we can see the arrow, only when we apply light. To get the stroboscopic effect, the light is made to fall on the arrow which is controlled by variable frequency. Such a type of lighting device, which uses a variable frequency system, is called as a strobe light. So by changing the frequency of the light we can change the flickering time of the light. If we set the frequency for 50Hz, the light will get ON and OFF by 100 times in one second, and if we set the frequency to 60Hz, the light will get ON and OFF by 120 times (by double). Suppose if we set the frequency rate at 0.5Hz, the light will get ON for 1time per second. If we show this frequency of light to the rotating arrow something extraordinary happens. The whole setup will seem to be held at a stationary position. But this is just an optical illusion. The arrow is still rotating at 1 time per second. But what causes the arrow to be stationary is called the stroboscopic effect.
The main reason for this optical illusion is the frequency of the light gets matched to the rotation of the arrow. As the light flickers for 0.5Hz (1time/second) of supply and the arrow rotates for 1RPS; during each flicker (i.e.,) during each flashing, the arrow will tend to rotate and complete its one rotation. As we can see the arrow only when we apply light; for one flicker the arrow will complete its one cycle. So it ends at the point where it starts. Again during the next flicker, the arrow completes its another rotation ending up at the same point. So every time as the light flickers, the arrow completes its cycle and remains at the same point. This is why the whole setup will appear to be stationary.
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Each rotation matches with the frequency of light flashing |
If we increase the frequency of the light to 0.75HZ, the light flashing will be faster than the speed of the rotation of the disk. So if we pass the light on the arrow, we can observe that the arrow will start to appear to rotate in backward. This is because the light will get flickered at a point before the arrow completes its one rotation. But as in the above case with a 0.5Hz frequency, the light flashes on the completion of one rotation of the arrow. So in 0.75HZ frequency, the arrow will complete only a part of its rotation. Let’s say it completes only ¾ of its rotation. So a quarter part is lost. Again during another flicker, the arrow completes only ¾ of its rotation, losing a quarter part from the previous ending point. So during each rotation, it loses its quarter part and appears to be rotating backward. If we decrease the frequency the opposite happens to the arrow by which it moves forwards.
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Light flashing is faster than the rotation of the arrow |
We can observe this effect in our day-to-day life. We can observe this in the car wheels and helicopter blades. In cars, during de-acceleration, the wheel appears to be rotating in a reverse direction. But there is no variable frequency light we are using in the car wheels to see the effect. This happens because of our eyes. Our eye has a frame rate of 1000 which means our eyes can capture 1000 frames per second. So when the rotation of the car wheel is less than the frame rate of our eyes, the wheel appears to be rotating in reverse condition.
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