Quantum Mechanics demystified, a try
People naturally think of light in connection to their vision. Common understanding is that light is a beam or a ray traveling in the air. Produced e.g. in the Sun, Traveling through space to Earth, reflecting from various objects around us, eventually reaching our eyes. Caught by cone cells and transferred to our brain. Where this one ray adds up in the view of our surroundings rendered in our brain.
That’s not incorrect. A bit simplistic though.
Quantum-mechanically light is composed of a bundles of energy packets called photons.
Photons propagate in space as electromagnetic waves.
Photons are making up not only visible light, but all electromagnetic radiation.
Photons can exhibit properties of both waves and particles depending on the context. This duality can be observed in experiments like Double slit experiment’s single photon interference.
Photon is born e.g. when an electron in an atom emits one in connection of returning from excited state to lower energy level. While there are other ways a photon can be created, this way is in our focus here.
This can happen in Sun’s core (see Tunneling) or it can happen e.g. in your electric torch. In space (or air in the latter case), it travels like a wave, mostly without interruption, as the surroundings is mostly empty.
Electron got excited because of another photon had previously hit it. Photon’s energy caused the electron to jump to higher energy level. Photon got absorbed. Again, there’s other ways for an electron to get excited, but this is the one we are interested in here.
However, this excited state is not stable, electrons prefer staying on their normal lowest possible energy level in the atom. Thus, after a very, very short while, electron returns back to previous lower energy level. This causes another bundle of energy to be released in the form of a photon. A photon was emitted. Just to be absorbed again once suitable atom comes across.
This bouncing goes on and on everywhere where atoms and photons impact. It’s like throwing a ball against the wall. Ball flying through the air without bumping into anything and once reaching the wall jumps backwards. Only that the ball coming back would be a different one from that thrown.
Aforementioned energy levels are called orbitals. Different makes of atoms, elements, have unique electron configuration on these orbitals.
Amount of energy between orbitals is quantized, called quanta.
This quanta of energy is emitted in a form of a photon when excited electron returns back to lower orbital.
Energy of the absorbed photon has to be big enough to make an electron excite. It might be bigger than the excitation energy quanta, with any excess energy transferred to thermal energy, used to excite other electrons or alike. Electron can even be released as seen in photoelectric effect.
Each element has specific orbitals, typically at the higher energy levels, that are sensitive to participate in excitation. This makes each element to emit (and absorb) photons with characteristic frequencies.
Frequency of the emitted photon is determined by formula $E=hf$, where $E$ is energy between higher and lower orbital and $h$ is Planck’s constant.
These frequencies are kind of fingerprints of the element. By observing these frequencies element can be recognized.
E.g. atoms in blue paint have the electron orbitals such that electrons emit photons having wave length of 450nm or frequency of \(6.6*10^{14} Hz\) when de-exciting.
Once the photon then hits a cone cell in our eye (or a rod cell in twilight), the energy bundle’s journey carries on in our optic nerves. But that’s already another story.