why shadow of a glass oscillates under the rotating fan behind which there is a lightsource

Question

So today I was sitting in mess, and light coming to me was fluctuating mainly because of rotating fan which was cutting the light source just right above it. But then i observed that the shadow of the glass instead of appearing and disappearing, it was also slightly moving left and right. can somebody explain it to me why it was happening??

Answer 1

It is because the light source is not a point. When the fan blade starts to pass in front of the light source, it partially covers it, so the light beam center is moved forward. When the blade move to the center of the light source, light passes from both sides of the blade equally so the shadow is in the center position. When the blade is finishing the pass through the light source, the light beam center moves backward as the front side is covered by the blade.

Answer 2

The light source was flickering at 60Hz because it was either fluorescent or LED. The fan blades were spinning at a speed (in revolutions per second) which was a submultiple of 60Hz. The result: the blades were strobing against the light pulses.

If the light blink frequency and the fan blade speed were constant, why then would the blade strobing effect wobble back and forth? The most likely explanation is this: it's common for those big, slow turning ceiling fans to be slightly out of blade balance. If the fan is suspended from the ceiling on a rod mount, then the whole fan assembly will wobble around in a small circle as the out-of-balance blades rotate. when you combine the wobble with the blade rotation, and shine a flickering light through the blade arc, the light and shadow pattern would then wobble too.

A less-likely answer is that the 60Hz line frequency was shifting slightly up and down, which would vary the blink frequency of the light source slightly up and down. Since there's a lot of inertia in the fan, it would average out the shifts and stay at constant speed. the blink frequency shift would then cause the light & dark patterns to wobble slightly back and forth.