tv flashes
Turn on a tv set. Turn off the room lights. Hold up a finger
in front of the tv screen so that you see the screen behind
your finger. Wave the finger rapidly back and forth. Instead
of seeing one finger moving back and forth, you see 3 or 4
copies of the same finger. Why? A television's picture flashes
on and off 60 times each second. When the picture is on you
see your finger in one position. Then the picture goes off
while your finger is moving to another position. During that
time there is not enough light for you to notice the moving
finger. Then the tv picture comes on again making enough light
to see the finger in its new position. Your brain remembers
what it saw just 1/60 second before, so you see the finger
where it was and where its new position is.