MadSci Network: Physics

Re: About special optical effect (loop) of moving hand in front of PC screen

Date: Mon Oct 25 01:47:54 1999
Posted By: Matthew B. Weyerich, Technical Coordinator,ES&R Dept., CPI Corp.
Area of science: Physics
ID: 940070504.Ph

Greetings, Max!

I think what you've observed is closely related to what one sees in a 
darkened room where there is a fast strobe light flashing on and off. Your 
computer screen is like a strobe light!

Your screen is not "displaying" continuously. Instead, it is actually 
re-drawing, or, "refreshing" the picture you look at many times a second. 
(Nowadays, most monitors do this at least 75 times each second. Their 
"refresh rate" is said to be 75 Hertz [Hz], or greater. Mine is set at a 
slower 60 Hz, because that's fast enough I don't personally notice any 
"flicker".Then again, I've always been a bit "slow"!) Your eyes and brain 
use something called "persistence of vision" to blend the individual 
pictures together into a coherent picture. Your brain doesn't usually 
notice the flashing.

Try moving your hand in front of the monitor. If you move very slowly, you 
will not notice anything unusual. If you move more quickly, you will see 
"many images" of your hand as it moves. You may even notice some of these 
fleeting images seem "transparent". Try it away from the monitor, too. Use 
one finger. You won't see "many images", but you may see a "blur" depending 
on how fast your finger moves.

I'm pretty sure what you are seeing is the same "blur" you see when you 
wave a finger very quickly under continuous lighting. The difference here 
is probably your monitor, which is "flashing" and "interrupting" the blur 
you'd normally perceive. In one sense, your monitor is flashing like a 
strobe light, "freezing" the movement of your hand. In another, your hand 
is moving in front of a flickering light source which otherwise seems 
continuous. Your brain notices something isn't quite "normal" about this! 
Our brains and eyes interpret this in the way which will give us the most 
possible information: we see a "divided blur", or, a set of semi-distinct 
"frozen" images in what we'd otherwise perceive as normal "motion blur".

I've found this can be a fun thing to play with! Under continuous light 
sources you can wave a pencil loosely between your thumb and forefinger. It 
will appear to "bend" as you wave it more quickly. Now, try it in front of 
your monitor. The effect seems much more pronounced. ("Disco-bendy-pencil"! 

I hope I've helped with your question. If you wish to know more, please 
start with the links below. If you need more help, e-mail me at, and I'll do my best to assist you!

Your MadSci,


Refresh Rate Defined:

Monitors (See Slide 4):

A whole Web-book about visual perception:

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