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

Doppler Effect question

  1. Jun 10, 2005 #1
    Hi, Im sort of a new to physics. So far I've only been doing motion, up to the formula for average power. However I was reading something in Earth Science as I was going to help someone study for their regents test, you know how school is almost over and all. Then I read about the Doppler Effect. I must be wrong, but it seems to propose that Electromagnetic energy contains some sort of structural force and there in resisting motion. To my knowledge thats what would cause it bend or stretch due the position proximity of Earth. Where is the error of my thinking?
  2. jcsd
  3. Jun 10, 2005 #2


    User Avatar
    Gold Member

    It might be easiest to envision the electromagnetic wave in the same way as a sound wave, since the Doppler effect applies to both. There is a difference that I'll mention in a minute.
    As the source of the wave approaches, the wave is compressed in front of it. As it recedes, the wave is stretched out behind.
    With EM, the speed must remain at c (whatever that is in accordance with the medium of transmission), so the energy gain in approaching is expressed as an increase of frequency so that the wavelength can shorten while still maintaining the proper relationship among the 3 components. Going away, the wavelength stretches and the frequency decreases.
    Sound works the same way, except that rather than being a constant, the speed varies dependent upon the ambient temperature & pressure as well as the medium. It also does require a physical medium of some sort, whereas EM radiation does not.
    Since gravity saps energy from waves, proximity to a significant mass can effect a redshift in light trying to escape. For a black hole, the redshift becomes infinite.
    This is a preliminary explanation only, to tide you over until someone like SpaceTiger can supply you with something more proper.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook