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Homework Help: Destructive interfrence of waves

  1. Feb 4, 2008 #1
    [SOLVED] destructive interfrence of waves

    1. The problem statement, all variables and given/known data
    My teacher was saying that if you have a light wave or a sound wave that collided with another wave that was pi out of phase, you get destructive interfrence. If the two wave have the same amplitude, they cancel each other out. I was just wondering where does the energy go after they cancel each other out.

    3. The attempt at a solution

    My guess would be that the energy certainly cannot be destroyed due to the law of conservation of energy. So i would guess the energy changes to heat energy since most 'waste energy' goes into heat energy. But what happens in the microscopic level? If it is a light wave, do the photons just disappear or do they transmit infra-red instead of light?

    Can anyone check if the stuff i guess are correct? Thank You.
  2. jcsd
  3. Feb 4, 2008 #2


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    Homework Helper

    It is incorrect to say that the energy gets converted. Energy is still propagating the medium as usual, except that it can no longer be noticed or detected. Take for example a rope where you send a positive wave from the left while your friend sends a negative wave from the right. The waves will pass right through each other and you will feel his wave on your end and vice versa. No energy is being lost or converted into heat (ideally speaking).

    The same happens with light or sound or any other form of wave energy. Light is a bit more complex because of its dual wave-particle nature, so let us not discuss light in terms of photons when we are discussing its wave nature. In the example of sound, at the "microscopic level", the air molecules are feeling the tension/compression energy of both waves in opposite directions, so Fnet = 0, and no movement occurs.
  4. Feb 4, 2008 #3
    Thank you for the reply.

    But i don't really understand... If I use an example of a sound wave, before the destructive interfrence, the air molecules are vibrating and so have kinetic energy. If the wave is unnoticeable and the air molecules aren't moving after the destructive interfrence, then shouldn't they have no more kinetic energy?
  5. Feb 7, 2008 #4
    I read up on kinetic energy stuff. In wikipedia, it says 'The kinetic energy of an object is the extra energy which it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its current velocity.'

    This means that when the particles are moving, they have that 'extra' energy and so have more energy than the same particle at rest. Doesn't this mean that after destructive interfrence, the wave should have have less kinetic energy as the air particles are no longer moving up and down? (I am not sure if I am right about the kinetic energy thing)

    But if there is less kinetic energy, where does that energy go?
  6. Feb 7, 2008 #5
    In any situation, i can firmly say that the energy is not destroyed because energy can't be destroyed... So the energy must be either unchange or converted or transferred to somewhere else. Can someone please explain what is going on in the situation, i'm quite confused...

    Thank you.
  7. Feb 11, 2008 #6
    I found out the answer. The answer is that the energy is stored as potiential energy when destructive interfrence happens and so the potiental energy can be converted into (in the case of a sound wave) kinetic energy of the particles when constructive interfrence happens.
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