When you move your hand in front of a screen or light, why does it look like your hand is moving at a “lower frame rate” so to speak?

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When you move your hand in front of a screen or light, why does it look like your hand is moving at a “lower frame rate” so to speak?

In: Biology

Just a quip because I’m not sure if I’m allowed to add text to the post, I noticed this cuz I was moving my fingers underneath the scanner at my retail job.

Because the light source is probably not incandescent (i.e. an old-fashioned light bulb).

Fluorescent lights, computer screens, and many LEDs actually flicker at a fast rate (like 60x per second give or take). If you were to film these in slow motion with a 120fps or 240fps slow-mo camera, you would see them flickering off and on many times. Our eyes don’t see this flicker, typically because it happens faster than can be perceived.

But if you move your hand back and forth quickly, you can force the light to be unable to keep up with your movement so your hand isn’t lit consistently as you move it back and forth in front of a flickering light source. This makes it move in a way that’s noticeably different. Kind of like how strobe lights make everyone at dance parties look like they are moving like robots.

An incandescent light bulb doesn’t ever flicker (well it shouldn’t, anyway) since it’s just a piece of metal that’s glowing because it’s hot, giving off light. You would not see this same effect in that case.