Emission and absorption spectra of chemicals

If you bring some sodium salt e.g. table salt over a flame, you will get a yellow light. If you use a "cereal-box-CD-spectroscope", you will notice that the emitted light gives a spectrum (emission spectrum) consisting of two yellow lines. These are two specific wavelenghts corresponding to sodium. 

Spectra of stars

If you examine the Sun spectrum, you will find two dark lines at the specific wavelenghts for sodium. Light from the hot core of the Sun is absorbed by sodium in the atmosphere of the star before it arrives to us and those two specific wavelengths appear to us as if they are "subtracted".

The Electron as a wave - Wave function Ψ

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"?

From the gross to the fine structure of the atom: spin-orbit interaction

Where do the two yellow 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 and then emit energy while returning to the original one. However, it has two different choices as it can jump not to just one but to two different energetic levels!

From the fine to the hyperfine structure of the atom: Zeeman effect & its use in (Remote) Magnetometry

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

Larmor precession - Spin excitation (Spin flipping)

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.

Lasers - Optical pumping - Spin polarization with (polarized) light or magnetism

Due to its optical transition, sodium couples in a resonant fashion to light and so do all alkali metals. It is possible to energise a medium using light, in a procedure called "optical pumping". This is the principle of the laser. Using light or magnetism we can induce spin polarization of a medium for different applications.

Laser guide stars: why astronomers shoot lasers at the Universe - the sodium transition in astronomy

Why do stars twinkle? How can we take perfect images of stars from Earth similar to those by the Hubble telescope? By establishing a technique for light correction based on image processing of a known laser light signal.

 Atomic magnetometers - Optically pumped magnetometers - Measuring magnetic fields with atoms and light

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.

Applications of optical magnetometers in biomagnetometry: a Magnetoencephalography (MEG) wearable and DAPRA's magnetometer project for MEG and MCG (AMBIIENT)

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.

Remote optical magnetometry - Measurement of the Earth's magnetic field using mesospheric sodium and Laser Guide Stars (LGS)

The magnetometric sensitivity of the technique is in the nanoTesla range and can be enhanced as much as five orders of magniitude with satellites or rockets.