Recognitions:
Homework Help

## Finding the wavelength

 Quote by Pranav-Arora Is the highest energy level infinity? If so, then the lowest possible induced wavelength is 91.17 nm. But how would i calculate the possibilities for the electron falling back to a lower energy level and the corresponding induced wavelengths?
You seem to be missing something here, but I don't understand what it is you are missing.

The highest energy level is indeed infinity, but that level cannot be reached since there is not enough energy in the incoming photon.

Btw, I've been checking up on this problem.
As DiracRules stated earlier, the photon can only be absorbed if its energy matches one of the jump-energies of the electron (almost) exactly.
After that the most energetic photon that can be emitted is a photon of this same wavelength.
A less energetic photon would have a longer wavelength.

Recognitions:
Gold Member
 Quote by I like Serena You seem to be missing something here, but I don't understand what it is you are missing. The highest energy level is indeed infinity, but that level cannot be reached since there is not enough energy in the incoming photon. Btw, I've been checking up on this problem. As DiracRules stated earlier, the photon can only be absorbed if its energy matches one of the jump-energies of the electron (almost) exactly. After that the most energetic photon that can be emitted is a photon of this same wavelength. A less energetic photon would have a longer wavelength.
In this question, electron cannot jump to the second level too since the energy of the incoming photon is 12.1 eV.
Am i right?

Recognitions:
Homework Help
 Quote by Pranav-Arora In this question, electron cannot jump to the second level too since the energy of the incoming photon is 12.1 eV. Am i right?
Yep.

Recognitions:
Gold Member
 Quote by I like Serena Yep. The electron will jump to the 3rd level.

Recognitions:
Homework Help
Not enough energy in the incoming photon.
It would need 2 incoming photons, but only one can be absorbed at a time, after which a photon is emitted, before a new photon is absorbed.
 Recognitions: Gold Member But i am not able to understand why the answer is (a) option?

Recognitions:
Homework Help
 Quote by Pranav-Arora But i am not able to understand why the answer is (a) option?
An incoming photon is absorbed, making the electron jump from the first to the third energy level.
This electron falls back to the ground state and emits a photon of the same wavelength as the absorbed photon.
Doesn't that match with answer (a)?

Recognitions:
Homework Help
 Quote by Pranav-Arora But i am not able to understand why the answer is (a) option?
Where is your global view? If this were a test paper question, you should be solving it in about 1 minute.

If an atom is excited to an upper level, the energy of the emitted photon could be the same as the energy of incoming radiation - if the drop to ground state is done in one step - or smaller, if the drop back goes via lower levels.

Those lower energy emissions have a longer wavelength, so the SHORTEST wavelength [that is what we were asked about in the question] is the same as the incoming radiation.

Option (a) is the nanometre equivalent of the Angstrom description of the incoming radiation - so would be the answer.

Note: the only other possibility was that the incoming radiation was not an exact match for one of the excited state - in which case it would be elastically scattered, and we would see option (a) for that reason.

To paraphrase the question:

"Did you know that the energy of a photon given off by an excited atom is no higher than the energy of the incoming radiation exciting the atoms?" - in combination with "Did you know that minimum wavelength corresponds to maximum energy?"

Other than recognising that the Angstrom wavelength corresponded to one of the nonometre wavlengths, no calculations were necessary in the question [as befits the idea that you should have completed the question in one minute]

Recognitions:
Gold Member
Sorry for the late reply but i thought i would first go through our discussion.

 Quote by I like Serena An incoming photon is absorbed, making the electron jump from the first to the third energy level. This electron falls back to the ground state and emits a photon of the same wavelength as the absorbed photon. Doesn't that match with answer (a)?

 Quote by PeterO Where is your global view? If this were a test paper question, you should be solving it in about 1 minute. If an atom is excited to an upper level, the energy of the emitted photon could be the same as the energy of incoming radiation - if the drop to ground state is done in one step - or smaller, if the drop back goes via lower levels. Those lower energy emissions have a longer wavelength, so the SHORTEST wavelength [that is what we were asked about in the question] is the same as the incoming radiation. Option (a) is the nanometre equivalent of the Angstrom description of the incoming radiation - so would be the answer. Note: the only other possibility was that the incoming radiation was not an exact match for one of the excited state - in which case it would be elastically scattered, and we would see option (a) for that reason. To paraphrase the question: "Did you know that the energy of a photon given off by an excited atom is no higher than the energy of the incoming radiation exciting the atoms?" - in combination with "Did you know that minimum wavelength corresponds to maximum energy?" Other than recognising that the Angstrom wavelength corresponded to one of the nonometre wavlengths, no calculations were necessary in the question [as befits the idea that you should have completed the question in one minute]
What do you mean by "elastically scattered"?

Yes i know that the energy of a photon given off by an excited atom is no higher than the incoming radiation, but is the energy given off always equal to that of incoming radiation?

And yes i know that energy is inversely proportional to wavelength.

 Quote by DiracRules What I mean is: if you put in the equation $E_n=\frac{R_H}{n^2}$ the values of $E_n$ and $R_H$, is there an integer that fits well in $n$? If so, then the electron can get to the excited state, else it can't. That's all.
I still don't understand what should i put the value of $E_n$.

Recognitions:
Homework Help
 Quote by Pranav-Arora What do you mean by "elastically scattered"? Yes i know that the energy of a photon given off by an excited atom is no higher than the incoming radiation, but is the energy given off always equal to that of incoming radiation? And yes i know that energy is inversely proportional to wavelength.

Elastically scattered means the incoming photon comes back out without losing any of its energy - naturally it has the same energy , so same wavelength as when it went in.

No the energy of an emitted photon is not always the same as the incoming, could be less than the incoming photon - but only if the atom was excited to the 2nd or higher level. [I gather, from some computations in this thread, that in this case it actually gets excited to the 3rd energy level - important for you to realise that I did NOT need to know that in order to answer the question!!!]

OK so you knew that energy is inversely proportional to wavelength for a photon. In that case you should have been able to answer the question - if you had recognised what the question was asking!!

The question asked "what is the shortest wavelength photon emitted?".
The inverse expression between energy and wavelength means that question could be re-written as "what is the largest energy photon emitted?"

Since you knew that any emitted photon would be the same, or lower energy, you should have recognised that you were after the same photon that went in. - Option (a)

Arguably, the question is really testing whether you can convert Angstroms to nanometres!

NOW, had all the options been longer than the incoming radiation, you would have had to work out which energy level the atom could be excited to [apparently the 3rd level], then calculate the energy and wavelength of the radiations for drops to intermediate levels to make you selection.

As I said before: that would make it a 5-10 minute question rather than a 1 minutes question - so inappropriate for the multiple choice sections of most tests/exams

Recognitions:
Homework Help
 Quote by Pranav-Arora Why doesn't the electron jump to the second level?

 Quote by Pranav-Arora In this question, electron cannot jump to the second level too since the energy of the incoming photon is 12.1 eV. Am i right?
And you were right.

Recognitions:
Gold Member
 Quote by I like Serena You already noted before: And you were right.
So why it jumps to the third level?

Recognitions:
Homework Help
 Quote by Pranav-Arora So why it jumps to the third level?
The difference in energy between the first and the third level corresponds (almost) exactly to the energy of the incoming photon.
Btw, only when there is an (almost) exact match will the photon be absorbed.

Recognitions:
Homework Help
 Quote by Pranav-Arora So why it jumps to the third level?
Don't forget that it is all but irrelevant that it jumps to the 3rd level!

Recognitions:
Gold Member
 Quote by I like Serena The difference in energy between the first and the third level corresponds (almost) exactly with the energy of the incoming photon. Btw, only when there is an (almost) exact match will the photon be absorbed.
The energy difference between the first and the third level is 10.2eV but it doesn't match with the energy of the incoming photon.

Recognitions:
Homework Help
 Quote by Pranav-Arora The energy difference between the first and the third level is 10.2eV but it doesn't match with the energy of the incoming photon.
I didn't calculate or check the energy of the energy levels or the energy of the incoming photon.

However, you already calculated before that the wavelength corresponding to the first and third level is (almost) equal to the wavelength of the incoming radiation.

Recognitions:
Gold Member
 Quote by I like Serena I didn't calculate or check the energy of the energy levels or the energy of the incoming photon. However, you already calculated before that the wavelength corresponding to the first and third level is (almost) equal to the wavelength of the incoming radiation.
Sorry it's my mistake, its not 10.2 eV.
But i still don't get why (a) is the minimum wavelength?