absorption spectra

Discussion in 'Physics' started by mentaaal, Mar 28, 2008.

  1. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
    Hi guys, i am having a particularly annoying night. I am having what could be considered, the opposite of an epiphany! I am trying to understand absorption spectra. Emission spectra is simple by comparison in that atoms can only emmit radiation from certain jumps pertaining to the makeup of the atom. But what i am not understanding here is why atoms only absorb certain frequencies in the absorption spectra models? In photoelectric effect experiments we did in physics, the higher the frequency of the incident light on the photocathode, the higher the kinetic energy of the emitted electron. So howcome, in absorption spectra, higher frequency photons dont get absorbed and pass through unimpeded? There is something i am clearly missing here!

    This line of thought led me to think about how is it that some materials transmit light and some dont. I have spent over an hour googling this stuff and its not really helping a whole lot.

    One more question, how does light propagate through a transparent material?

    Thanks in advance!
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
    Therein lies the crux of your query.

    For emission or absorption spectra electrons do not get emitted. They remain within their atom/molecule. Emission and absorption spectra are really opposite sides of the same coin.
    When energy of the right frequency is emitted an electron drops to a lower orbit and we see the emission as light of a certain fequency. Thus we see bright lines on a dark background for a continuous frequency spectrum.
    This process is reversible so the atom can absorb energy (light) at the same frequency and allow the elctron to jump up to the higher orbit. this we see as dark lines on a bright background of a continuous frequency spectrum of supplied light.

    The photoelectric effect is something quite different. Here nothing happens as the supplied energy increases until the energy supplied is sufficient to remove the electron from the atom entirely. Once this critical energy is exceeded any higher levels of supply merely accelerate the electron away faster.
  3. beenthere

    Retired Moderator

    Apr 20, 2004
  4. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
    Hey thanks for the replies you guys, problem solved!