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    Can someone explain this question to me:

    Line spectra were observed before they could be explained by theory. We now know that photons of characteristic frequency are emitted when vapour of an element is bombarded by energetic electrons. The spectrum of the light emitted contains lines, each of a definite wavelength.

    Explain how:

    the bombarding electrons cause the atoms of the vapour to emit photons
    (This half of the question I can do easily)

    The existance of a spectrum consisting of lines of a definite frequency supports the view that atoms have discrete energy levels.
    (I really don't understand this)

    I would be really grateful if someone could explain the second part of the question to me. I've looked at the mark scheme but it hasn't helped at all.
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    Hey, I was just doing this paper yesterday actually.

    Ok, so the mark scheme says: "photons of characteristic frequencies emitted from atoms of a particular element"

    An atom has discrete energy levels. This means it can only have certain amounts of energy. So lets say it can have "10 amounts" of energy, and "20 amounts" of energy. The atom wont be able to have "15 amounts" or anything between 10 and 20 actually. (these are arbitrary units of course)

    So if an atom has discrete energy states, it means that the photons emitted during de-excitation can only have certain energies. The energy of the emitted photon has energy equal to the difference between the 2 energy levels. E1 - E2 = hf

    If a photon can only have certain energies, it can only have certain frequencies. E=hf. (h is constant)

    In line spectra, each line corresponds to a particular wavelength of light/(photons). As these photons can only have certain energies/frequencies it must be the same for wavelength.

    So you can look at all these different wavelengths and then calculate which element it is from.


    That's what I understand from the line spectra topic. You only have to learn the key points which is given in the mark scheme and the text book but hopefully this has helped. Any questions just ask
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    Thank you I seem to find it really hard writing explanations for this topic. What would you write for your answer?
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    & one more thing please. Do you know why the kinetic energy of an emitted electron has a maximum value?
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    Explain how:
    The bombarding electrons cause the atoms of the vapour to emit photons.

    The existence of a spectrum consisting of lines of a definite frequency supports the view that atoms have discrete energy levels.

    Bombarding electrons collide with atoms of the vapour, transferring its energy and so the atoms become excited. This makes the electrons move to a higher energy level. Electrons in the excited state are unstable and so cascade back to the ground state(de-excitation). During this process the electrons lose energy and emit a photon at the same time.
    The energy of the photon is equal to the difference in energy levels. The frequency of the emitted photon is given by E1-E2=hf. Atoms have discrete energy levels and since only certain photon energies are allowed, you only see the corresponding wavelengths in the line spectrum.
    (something like this will get you 5-6 marks)


    Do you know why the kinetic energy of an emitted electron has a maximum value?

    The electrons in the metal all receive the same energy from the incident photons.(E=hf)
    But the work done by the electron to get the the surface may be different.

    if you look:
    Ekmax = hf - (workfunction)
    h is always the same
    f is the same
    so workfunction must be different
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    Thank you
 
 
 
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