# I don't get it

## Main Question or Discussion Point

So yeah I am sitting here, and I should be studying for my French test but I keep moving my pencil back and forth, and it is pissing me off that I don't understand why it appears as if I see the pencil in different places at the same time. <---that was a long sentence. Or actually that I see where the pencil used to be before...

So I can't imagine that there is a delay in the light travelling to my eyes because thats just ridiculous light travels way to fast. Is there a delay in the eyes registering the image, and sending it to the part of the brain where you see?

Related Other Physics Topics News on Phys.org
LeonhardEuler
Gold Member
Mozart said:
So yeah I am sitting here, and I should be studying for my French test but I keep moving my pencil back and forth, and it is pissing me off that I don't understand why it appears as if I see the pencil in different places at the same time. <---that was a long sentence. Or actually that I see where the pencil used to be before...

So I can't imagine that there is a delay in the light travelling to my eyes because thats just ridiculous light travels way to fast. Is there a delay in the eyes registering the image, and sending it to the part of the brain where you see?

Yes, there is what is called the permanence of vision. An image stays in view slightly longer than the time for which light is actually shining on the eye. Actually, the lightbulbs in a normal room go on and off 120 time every second due to the alternating current they recieve (they don't fully turn off in this short time, but they do turn off enough that the difference between the maximum and minimum illumination would be easily visible if they lasted longer. An LED can turn completely on and off 120 times a second and if you looked at it, it would just seem on) The time this takes is about 1/60 of a second.

Just like if I tapped a stick on the ground at a very high frequency I would hear a non stopping sound. Thanks.

HallsofIvy
Homework Helper
Are you perhaps sitting in front of a television set while doing your homework? (A very bad idea by the way!) Or are you doing your homework on a computer? Because a television set, or computer, refreshes the screen at regular intervals, it sets up a "strobe" effect. If you wave your pencil in front of the screen, you get more light from the screen at that times. While you don't notice it normally, waving a pencil in front of the screen will cause you to see the pencil more clearly at those moment so instead of a "blur" you see the pencil itself at several different moments. That together with the "Persistance of Vision" LeonhardEuler explained makes it look like you are seeing more than one pencil at one time.

like with a fan, interesting point...

Haha, yes my computer was directly in front of me and my television was behind me.

Last edited:
Mk
HallsofIvy said:
Are you perhaps sitting in front of a television set while doing your homework? (A very bad idea by the way!) Or are you doing your homework on a computer? Because a television set, or computer, refreshes the screen at regular intervals, it sets up a "strobe" effect. If you wave your pencil in front of the screen, you get more light from the screen at that times. While you don't notice it normally, waving a pencil in front of the screen will cause you to see the pencil more clearly at those moment so instead of a "blur" you see the pencil itself at several different moments. That together with the "Persistance of Vision" LeonhardEuler explained makes it look like you are seeing more than one pencil at one time.
http://www.aapt.org/Contests/pc05full.cfm?Meeting=sm05&Category=Contrived&Placing=42
Here is an interesting picture describing the same kind of thing.

Gir
Mozart said:
So yeah I am sitting here, and I should be studying for my French test but I keep moving my pencil back and forth, and it is pissing me off that I don't understand why it appears as if I see the pencil in different places at the same time. <---that was a long sentence. Or actually that I see where the pencil used to be before...

So I can't imagine that there is a delay in the light travelling to my eyes because thats just ridiculous light travels way to fast. Is there a delay in the eyes registering the image, and sending it to the part of the brain where you see?

yes there's a delay, but there's also the problem of your eyes only being able to see about 60 frames per second, anything moving faster than that gives us a false perspective.

HallsofIvy said:
Are you perhaps sitting in front of a television set while doing your homework? (A very bad idea by the way!) Or are you doing your homework on a computer? Because a television set, or computer, refreshes the screen at regular intervals, it sets up a "strobe" effect. If you wave your pencil in front of the screen, you get more light from the screen at that times. While you don't notice it normally, waving a pencil in front of the screen will cause you to see the pencil more clearly at those moment so instead of a "blur" you see the pencil itself at several different moments. That together with the "Persistance of Vision" LeonhardEuler explained makes it look like you are seeing more than one pencil at one time.
That reminds me. Have you ever tried plucking a guiter string or a string of a bow in front of a TV? You can actually see the waves rippling through the strings instead of the normal apparant back and forth motion you would see without the TV in the back.

DaveC426913
Gold Member
HallsofIvy said:
Are you perhaps sitting in front of a television set while doing your homework?
Or under any fluorescent lighting?

Gir said:
yes there's a delay, but there's also the problem of your eyes only being able to see about 60 frames per second, anything moving faster than that gives us a false perspective.
It's more like 10 frames per second. Otherwise, movies, at 24fps, would look like a bunch of still images to us.