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I'm having a bit of trouble understanding Balmer lines, is this correct, and the questions are in brackets:

1) A star needs to be hot enough such that electrons are in the n=2 state (why does the temperature cause the electrons to be in n=2 state?)

2)As light of a specific frequency passes through the star's atmosphere it excites these electrons in the n=2 state raising them to higher energy levels (why can't it excite electrons not in n=2?)

3) The electrons the de-excite and drop back down to n=2 (why can't say and electron originally in n=3, drop down to n=2 without being excited in point 2)

4) The lose of energy is emitted as photons (E=hf) in random directions, and possibly in multiple steps (what is the correct phrasing for this, I mean perhaps two photons will be emitted from n=4 to n=3 to n=2 and so won't be the same frequency as the specific frequency in point 2 which correlates to Balmer lines)

5) this results in reduced intensity at the specific that correlate to Balmer absorption lines.

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1, Higher electron levels require energy input to raise the electrons to that level. In a star that ultimately comes from heat.

2, It can but that would give light of a different energy/frequency

3, It has to have been excited at some point. The electron doesn't stay in the excited state for very long without decaying so we generally consider the input excitation and the output decay as part of the same process.

4, The Balmer line series is any decay down to n=2. You could have a single Balmer line (H-β) from n=4 to n=2 or a lower energy n=4 to n=3 Paschen line, then a Balmer (H-α) from n=3 to n=2

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  • $\begingroup$ #2 was asking about absorption, not emission, so the answer would involve what frequencies (visible) are available already, i.e., what frequencies are making their way through the outer layers of the sun. $\endgroup$ – Ben Crowell Jun 17 '13 at 0:09
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  1. There are two ways to excite a Hydrogen atom to the n=2 level, either with a UV photon or by inputting that energy as heat (why it only occurs in the hot stars of OBAF class)

  2. Visible photons only have certain energies (E=hf), as the energy levels in an atom are discrete if the photon does not have enough energy it cannot excite the electron to a higher energy level. Photons in the visible spectrum do not have enough energy to excite electrons in hydrogen to the n=2 level as it is such a big gap.

  3. Electrons could drop lower than n=2 but the photon emitted would not be in the visible spectrum, it would not influence the Balmer absorption lines at all and hence is not considered.

  4. Electrons 'cascade' through the different energy levels, causing photons of many different frequencies to be emitted. Only a small number of these deexcitations cause photons of the original frequency to be emitted, and these photons are radiated in all direction causing their intensity to be distinctly decreased

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