Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What keeps EM waves distinct?

  1. Dec 12, 2004 #1
    Electromagnetic waves interfere with each other.
    They cancel each other out.
    They can be complimentary or destructive, but they do interact with and change each other.

    If that's the case, with the plethora of electronic devices that surround us and the endless radio traffic we incessantly produce, why don't they all simply muddle each other and make them unusable?

    Even signals of different frequencies can affect each other.

    What am I missing?
  2. jcsd
  3. Dec 12, 2004 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    While they superpose to create a unique sum, the individual components remain distinct. So they do not effect each other but only the sum total of EM energy present.
  4. Dec 12, 2004 #3


    User Avatar
    Science Advisor
    Homework Helper

    What you are missing is that most electronic devices are designed to operate with a "signal" that far exceeds the noise level which is comprised of all that other stuff out there! If the noise level were raised sufficiently then they wouldn't function.

    Try placing a portable radio close to your computer and listen to an outlying station (even a local station will do if it's not one of those 50,000 W power stations!). Report back to us when you're done. :-)
  5. Dec 12, 2004 #4
    I understand that.
    If that weren't true, then it wouldn't work.
    What I am failing to grasp is how that is true.

    Of course.
    Why isn't the noise too loud, though?
    Look at cell phones in NYC.
    There can be thousands of people talking on cell phones that operate in the same narrow band of frequencues at the same time in the same close vicinty at the same approximate power all in a city that is inundated with radio signals incessantly.

    Electromagnetic waves affect each other in basically the same way that waves in a pool of water do.
    If I make a wave in phase with an existing wave, the first wave gets bigger, they are no longer distinct entities.
    If it is out of phase, they cancel each other out, and whichever wave had a greater amplitude is not decreased by the amplitude of the second wave, and the second wave disappears.
    It works the same way with color, correct?

    Why don't the waves simply blend?
  6. Dec 12, 2004 #5


    User Avatar
    Gold Member

    "If I make a wave in phase with an existing wave, the first wave gets bigger, they are no longer distinct entities."

    You can't really make two waves in phase with each other. If they have the same source, the same direction and are in phase, then you're really just increasing the amplitude of the first one.

    But - if you create two waves that are different in only one (or more) of those properties, they will exist completely independent of each other. If you wait for a moment, you will see the two waves pass each other and move on.

    There is a grade 11 Physics experiment that demonstrates exactly that with a spring or string. Start start two waves along it, the first one will get to the end and bounce back. It will encounter the second, and they will pass each other and keep moving as if they never encountered each other.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook