A 12.5 eV electron beam bombards gaseous hydrogen at room temperature. The number of different spectral lines emitted is:
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Question

A 12.5 eV electron beam bombards gaseous hydrogen at room temperature. The number of different spectral lines emitted is:

G1 Physics · Accepted Answer

The ground-state hydrogen atom requires 10.2 eV to reach n=2 and 12.09 eV to reach n=3. A 12.5 eV incident electron can therefore excite hydrogen up to n=3. The number of possible emission lines from levels 1, 2 and 3 is N = n(n-1)/2 = 3×2/2 = 3 Hence option C follows.