Ιf light or the photon is considered as both particle and wave could the same apply to the electron? If a water wave is water spread in space, could the electron be considered as charge/energy "smudged" in space? The Schrödinger equation and the wave function Ψ provide a model for the related probabilities. The quantum numbers are the solution of the equation. They are related to the rotational motion from classical mechanics! Is this why we say "quantum mechanics"?

Where do the two spectral lines of sodium come from? The electron in the outer shell of the sodium atom can absorb energy and transition to a higher energetic level. However, it has two different choices as it can jump not to just one but to two different energetic levels!

The electron in the outer shell of the sodium atom can absorb energy and make two different energetic jumps; in the presence of a magnetic field it has even more energetic jump choices! We refer to magnetic sublevels.

Spins of atoms are similar to little magnets. If they are static, when placed in a magnetic field they will align with it. If they are rotating around the nucleus in an atom, they will precess or "wobble" around the direction of the magnetic field with a specific precession frequency (Larmor precession frequency).

If we provide electromagnetic radiation of this frequency, we will mediate a transition from the aligned to the anti-aligned state.

Because spins are similar to little magnets, they can be used as magnetic field gauges, as magnetometers. Due to Larmor precession and optical pumping, it is possible that changes induced in an atom population are proportionate to the magnetic field intensity.

The new MEG wearable measures the magnetic field of the brain using an array of optically pumped magnetometers. They consist of cube sensors filled with rubidium (Rb), an alkali metal (like sodium) in gas form. The Rb atoms are excited with a 795-nm circularly polarized laser beam, tuned to the D1 transition of Rb.