# Question Video: Determining the Most Populous Energy Level in a Laser’s Active Medium Physics

The active medium of a laser contains atoms that have energy levels corresponding to the ground state, metastable state, and excited state of electrons. When the medium is at equilibrium, with no energy being supplied to it from an external source, which of these states will have the highest relative density of electron occupation?

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### Video Transcript

The active medium of a laser contains atoms that have energy levels corresponding to the ground state, metastable state, and excited state of electrons. When the medium is at equilibrium with no energy being supplied to it from an external source, which of these states will have the highest relative density of electron occupation?

Here, we’re considering what’s called the active medium of a laser. This contains atoms, we’re told, that have three separate energy levels: the ground state, the metastable state, and the excited state of electrons. These energy levels will appear in this order, the ground state at the bottom with the lowest energy level, then the metastable state above that, and the excited state above that. The ground state, of course, is always the lowest energy level. And this atom in the active medium contains an excited state above the metastable state in order to make what’s called a population inversion achievable. This occurs when there are more electrons in some state out of the ground state than in the ground state.

As a starting or baseline condition, we can picture any electrons in our atom as being in the ground state. This is the lowest energy state of the system overall and therefore the one towards which it naturally tends. If we wanted to use our active medium to create laser light, we would send in photons with a particular frequency so that, upon absorption, they would excite electrons from the ground state to the excited state. These excited electrons would then fairly quickly decay down to the metastable state where they would remain for some time and ideally allow for the creation of a population inversion.

In our system though, we’re told that no energy is supplied to it from an external source. Because there is no external energy supply, there is no reason for any of these electrons in the ground state to leave that state. We can expect, then, that without supplying any energy from an external source, the relative density of electrons in the ground state will be greater than the relative density of electrons in any other state in the atom. We see then that in order for lasers to work, some energy must be supplied from an external source.