# A question on the emission spectrum and absorption spectrum

I am submitting this question because I am confused on how light is absorbed and emitted from atoms and how that relates to the colours we see. I have tried reaseaching the answer to my question but haven't had a solid answer. So I decided to ask on this forum.

So my question goes like this:

If I was to passed an electric current though a material with atoms that have electrons that absorb some engery equal to the energy difference between the energy levels then that electron would jump to the next energy level but only stay there very briefly and fall back down to its original energy level emitting a photon with the same amount of energy that was absorbed. And since the energy of a photon is directly proportional to the wavelength of that photon, the material would only emited certain wavelengths of light given by the emission spectrum which may look like this:

So no light is coming in (just electricity) and only certain light is coming out. Those wavelengths of light would then combine to form the colour of that object.

Makes sense so far and I assume this how bulbs work.

Another way to get atoms to emit light is to shine white light on an atom and the electrons would absorb the photon if the energy of that photon was equal to the energy difference between the energy levels. And the electron would jump to the next energy level and be absorbed. All other wavelengths do not have sufficient energy to allow an electron to jump to the next engery level so they will pass though the atom unchanged.

So the absorption spectrum will look like the following:

So light of all wavelengths goes in and only light which is not absorbed comes out.

Now since the electrons fall back down to their original engery level like in the first example, they will emit a photon with the same amount of energy that was absorbed. And since the energy of a photon is directly proportional to the wavelength of that photon, the atom will emit light with wavelengths that are the same as the light which was absorbed.

So you will see all the wavelengths of light be emitted from the atom because the light with insufficient engery passes straight though unchanged and the light which does have enough engery gets re-emitted anyway after it was absorbed. So i'm confused.

Plus none of this reflects what real life objects do because they only emitted the light we see and absorb all other light, not have them pass though unchanged.

Two small other questions:

1). In the first example the electrons moving with current gives energy to the electron in the atom. So the electron in the atom absorbs the moving electron? If so how is this possible because they are both negative?

2). Why is it that the electron loses energy when it jumps to the next energy level?

Someone please explain!

Thanks!

## 1 Answer

Another way to get atoms to emit light is to shine white light on an atom and the electrons would absorb the photon if the energy of that photon was equal to the energy difference between the energy levels. And the electron would jump to the next energy level and be absorbed. All other wavelengths do not have sufficient energy to allow an electron to jump to the next engery level so they will pass though the atom unchanged.

What is unclear to me is what you mean by being absorbed. As I say below, an electron cannot be absorbed, which is what I think you are implying above, but a photon, as the force carrier between electrons, can be absorbed and emitted.

I think there are duplicates for the other related questions in your post, so I will stick to the last two in this answer.

Why is it that the electron loses energy when it jumps to the next energy level

Let's take the common usage of the word jump as upwards. So in this above case, the electron gains energy. It loses energy when it falls back down to a lower level.

I have to admit that I don't like using words like jump and fall, because they are based on the Bohr model of the atom, which is not correct in almost every aspect.

So let me give you two pictures, one of the old model, which your question is based on, and one of the more modern picture.

The Bohr model (of 100 years ago)

The Orbital Distribution Density model

The electron will tend to lose energy if it can, by emitting a photon of the correct wavelength, that enables it to transition to a lower energy level, but if that lower level is already occupied to the maximum amount, then the electron is forced to stay at a higher level.

The difference between the pictures is the the Bohr model assumes a particle structure, whereas we now think in terms of the probability of finding an electron in a certain region, so we cannot be as definite as in the earlier model. Also, when the transition from one level to another occurs, it is not a smooth transfer like a car changing lanes, it is for a time a more chaotic operation, with the electron (or rather its' likelyhood of being found) bouncing around the place until it settles into a lower orbit.

In the first example the electrons moving with current gives energy to the electron in the atom. So the electron in the atom absorbs the moving electron? If so how is this possible because they are both negative?

There is no question of an electron absorbing another electron. Instead, by means of photon emission, momentum can be transferred between electrons, bearing in mind the conservation laws regarding energy and momentum.

An example of this is a Feynman Diagram:

Where the wavy line represents energy and momentum being transferred by means of a photon.

• Thank you for your response. I will further reasearch on Feynman diagrams and the orbital distribution density model. Please can you link me the duplicates you are talking about. Thanks Commented Feb 26, 2017 at 19:54
• Hi William, I feel this below is related regarding absorbed light , but if you are not happy I will try an answer, there is no need to spell out why it's not for you, I will just go on what is in your post, and do my best to answer it.: physics.stackexchange.com/questions/165996/…
– user146020
Commented Feb 26, 2017 at 20:05
• Regarding the answer to the question in the linked post. It says that the photon is re-emitted but in a different direction so the detector does not pick it up and you get the dark lines. But this does not make sense because real objects do not behave this way. They only emit (aka reflect) the light we see. If the answer was the case for real objects two people standing in different directions would see different things because light is re emitted in different directions. Commented Mar 14, 2017 at 14:31
• Yes, there are two things to consider 1. The quantum world has to match up with our classical observation, Bohr's correspondence principle, to stretch things a bit), which is Snells' Law for reflection, and 2. AFAIK in QFT, Fermats principle of least time is incorporporated in Feynmans QED scattering amplitudes which is well worth a slow careful read, as again it is Snell's law in more advanced math.
– user146020
Commented Mar 14, 2017 at 15:56