Q: What is the purpose of having two LEDs? Do they both blink exactly the
same
   way? Or are they out of phase somehow? Why not just have one LED?

A: When the string vibrates with the correct frequency, two lines are
projected=20
   at a fixed position on the string. If frequencies don't match, the lines

   appear to be blinking or moving to the observer's eye. The moving or
blinking=20
   effect decreases with decreasing difference between the two frequencies.





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