# B Light determination for emission spectrum

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1. Feb 26, 2017

### Klupa

Hi all, the title is probably a bit confusing but I was wondering.
What determines the type of light emitted when electrons move down electron shells and emit energy? Why isn't all the light emitted the same colour?

2. Feb 26, 2017

### Staff: Mentor

The color, or more appropriately the frequency, of the emitted light is directly related to the energy of the photon. $E = h \nu$, where $\nu$ is the frequency and $h$ Planck's constant. By conservation of energy, the energy the electron-nucleus system loses results in the generation of a photon with exactly that same energy.

3. Feb 26, 2017

### Staff: Mentor

If there is just a single possible transition, all light will have the same color. That is rarely the case, lasers are the most notable example.

4. Feb 26, 2017

### Klupa

How are lasers an example of this?

5. Feb 26, 2017

### Staff: Mentor

Laser light comes from a single transition between two energy levels. It is monochromatic.

There are exceptions, but those are beyond the scope of this thread.

6. Feb 26, 2017

### blue_leaf77

In addition to the frequency or color mentioned by the others, the coherency can also be one of the factors discriminating between the so-called spontaneous emission and stimulated emission. In the former, the emitted photons are incoherent whereas in the latter they are coherent which is why lasers work the way we know it today.
When only two levels are involved in the transition, ideally the emitted light will have only one color. But due to the uncertainty in energy which arises the moment the atom interacts with EM radiation, the emitted light will have slightly broadened spectrum, i.e. it contains other colors with lower intensity than that which corresponds to the energy difference between the two levels.