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B Add two functions, same frequency to produce one greater?

  1. May 20, 2017 #1
    Is their any way to add two wave functions like sin or cos in such a way that you could double the frequency or at least increase it?
     
  2. jcsd
  3. May 20, 2017 #2

    jedishrfu

    Staff: Mentor

  4. May 21, 2017 #3

    mfb

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    2016 Award

    Staff: Mentor

    You can follow the definition of a frequency to see that every sum of functions of the same frequency has the same frequency again.
     
  5. May 21, 2017 #4
    ^That was my personal conclusions as well. I ask because I was wondering if their was a way to increase the frequency of the E&M waves by combining them some how.
     
  6. May 21, 2017 #5

    mfb

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    Staff: Mentor

    Well, you can play some tricks.

    Define f(x)=sin(x) for [0,pi], [2pi,3pi], [4pi, 5pi] and so on, f(x)=0 otherwise.
    Define g(x)=-sin(x) for [pi,2pi], [3pi,4pi], [5pi, 6pi] and so on, g(x)=0 otherwise.
    Both functions have a period of 2 pi.
    Define h(x) = f(x)+g(x) = |sin(x)| which has a period of pi as h(x+pi)=h(x) for all x. It also has a period of 2 pi as h(x+2pi)=h(x) for all x.

    To analyze this properly, you can use Fourier transformations of the functions. They add nicely - a frequency component that is present in the sum has to be present in at least one of the summed functions. This is also true for electromagnetic waves, you cannot create higher frequencies simply by having two beams illuminate the same place.

    In matter, there are exotic effects which can lead to higher electromagnetic frequencies. This is known as upconversion.
     
  7. May 22, 2017 #6

    jbriggs444

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    You could take +sin x and -sin x. Upon adding them you would have a function with every frequency.
     
  8. May 22, 2017 #7
    Thanks for the replies, they are really great.
     
  9. May 22, 2017 #8

    WWGD

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    If the ratio of the periods is Rational, then the sum of periodic functions is periodic with period the "LCM" .
     
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