THE STROBOSCOPIC EFFECT

When a running fan is viewed under tube light, it appears to rotate backwards or forwards.

Our houses are supplied with alternating current of frequency 50 cycles per second. That means, the current is 'on' for 50 times a second and 'off' 50 times a second.

Our eyes has the property called 'persistence of vision'. It means, whenever we see an object, it's image is retained in the eye for one tenth of a second. No image in the eye vanish immediately. Due to this property, our eyes do not perceive flickering of the electrical lamp which arises because of alternating current.

Say, now you are viewing a table fan under electrical lamp. When light (current) is 'on', a particular image of the fan blade is recorded in our eyes. When the lamp is 'off' for some milli- seconds, the fan blade might have rotated by an angle to a new position. The new position of the blade may be in forward or backward to the original position depending on the speed of the fan. When the light comes, our eyes see the new position of the blade and perceive it as rotating forward or backward. This is called stroboscopic effect.

Note 1. This effect does not occur in natural light source like sun and candle which emit light continuously.

Note 2. When a fan is running at the speed of 50 rotations per second, it appears to be stationary under electrical lamp. If the speed is different from 50 rps, the blades appears to rotate slowly forwards or backwards depending on the speed.

So the fan and lamp teach you 1. alternating current 2. frequency 3. persistence of vision which is made use in the making of the movies. 4. rotation speed 5. relative speed and 6. illusion.

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