Why Is there a minus in electron volt requirement of electrons?

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Offline Nicholas Lee

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Is the a plus to signifying above 0, or not.?
I read " Most glass begins absorbing light significantly at about 350 nm and below (that's 3.5 eV). Very high purity fused quartz is transparent down to about 200 nm (6.2 eV)."
Then I looked at the hydrogen atom energy levels/shell levels, and the electron energy level requirements were in - minus, like -13.6, and -3.6 for the electron to move to a higher shell level.
But in the statement about glass there is no minus, in 3.5 eV.
Does this mean its above minus, and it is plus +3.5 eV like this.
So it would mean there are no ranges in the hydrogen atom to absorb ultra violet.
Is this correct, or wrong, still studying about electrons right now, its complicated.
I am grateful for tour help, anything helps even a few words.
« Last Edit: 21/07/2016 21:25:16 by chris »


Offline alancalverd

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It's a sign convention that you may find useful later. The energy level of a completely free electron is 0, and becomes negative as it approaches the nucleus. The value of the convention is that to promote an electron from an inner shell to an outer one, you have to add positive energy  (-13.6) + (+10)  = (-3.6), so you need at least  10 eV of photon interaction to make the transition. Equally, in returning to the unexcited state, the electron will emit a photon of 10 eV and the atom will lose 10 eV of energy, so the electron energy level will become "more negative". This makes sense as photons cannot have negative energy!
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