Homework Help: Identify emission line spectrum

1. Oct 8, 2014

desmond iking

1. The problem statement, all variables and given/known data
refer to the question

2. Relevant equations

3. The attempt at a solution
from figure b , moving to the right, we know that the wavelength increases. since e= hc/λ , so the energy diffrenece should be increases as going to the right, so i consider line 1,2 and 3 is in a specific series , and 4,5 in another series. line 1,2,3 should have higher energy difference , the question ask for electron transition ftom E1 to E0 . So , my ans is line 3 . but the line 3 is not choice for this question. the ans is line 5 . why?

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2. Oct 8, 2014

BvU

Hi Des. Posting goes against a whole lot of PF guidelines -- as you probably are well aware.
So I can't state here that I agree with your reasoning and the book answer must be in error.

3. Oct 8, 2014

Staff: Mentor

I would have said line 3, too.

4. Oct 8, 2014

rude man

Hard to say. There are 6 possibilities, not 5, but E1 to E0 and E3 to E1 are about the same. I guess both of these could be 3.

5. Oct 9, 2014

andrevdh

Yes the wavelength increases to the right, but the energy of the produced photons decreases to the right according to your formula since the energy is inversely proportional to the wavelength, that is the energy of the photons decreases as the wavelength increases. So the higher energy lines is on the left and the lower energy on the right. The key to the answer is in the remark rude man made. The wavelengths and energies of the two transitions he mentions are close together. Why? And which transition will give the shorter wavelength of the two?

6. Oct 9, 2014

Staff: Mentor

We are told it's the hydrogen spectrum. So we know the Lyman Series is well-separated from the Balmer Series. This would seem to leave no room for debate.

Last edited: Oct 9, 2014
7. Oct 9, 2014

ehild

The energy scale in the figure can be logarithmic. :)

ehild

8. Oct 9, 2014

desmond iking

so can someone draw a conclusion?

9. Oct 9, 2014

ehild

My conclusion is that the official answer is wrong as it happens very frequently.

10. Oct 9, 2014

rude man

I think ehild is right, the answer is wrong. It must be 3 if there IS a right answer.

11. Oct 9, 2014

Staff: Mentor

I agree with myself. Definitely 3.

12. Oct 9, 2014

BvU

It's physics, not some kind of referendum, for goodness sake. You've had four heavyweights backing you up !

13. Oct 10, 2014

Staff: Mentor

Was there a 6th line in the spectrum, one which got blotted out? Just wondering....

When Q doesn't match A, sometimes it's the question which is wrong. Not always is the textbook's answer at fault :w

14. Oct 10, 2014

BvU

No we are told these are some of the energy levels in the H atom.

It could well be the student only has the 1/n2 expression for the energy levels at this point in the curriculum, (he doesn't give any relevant equations at all) and in that case, from the ratios of the energy differences (assuming a linear scale, I admit), E0 in the picture is for n = 4 with reasonable certainty. Gives wavelengths in the micrometer range (2.2, 2.6, 4.1, 4.7, 7.5, 12.4). So even then, the wavelength picture doesn't fit qualitatively.

But for sure E1 -> E0 is the center of the five lines (where we still have to assume the sixth is off-scale to the right -- or cut off by the student :-) ).

Too many assumptions, pictures too vague. What book is this from ?

15. Oct 10, 2014

desmond iking

Why is it so? Thank you all mentor!!

16. Oct 10, 2014

Staff: Mentor

We are clearly told the line spectrum pertains to the hydrogen atom.

17. Oct 10, 2014

BvU

Why is what so ?

is what the text says. We are somehat brainwashed to think E0 = -13.6 eV, but the author can name the levels any way he wants -- if he is devious enough. I had fun measuring the y coordinates on my screen and found 0, 79, 124, 149 mm. Matches 0, 1/16-1/25, 1/16-1/35, 1/16-1/49 suspiciously well. in eV that would give 0, 0.306, 0.472, 0.572.

With
So $\lambda = {hc\over eE}$ if E is in eV (e = 1.6e-19 Coulomb), the wavelengths I came up with -- and that don't match the righthand Figure (b) :(