Blog of Veikko M.O.T. Nyfors, Hybrid Quantum ICT consultant

Quantum Mechanics demystified, a try

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What’s light

People naturally think of light in connection to their vision. In that context, 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 and 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 made of a bundles of energy called photons. Photons are quantums of energy representing not only visible light, but all electromagnetic radiation.
Photons can act like waves or particles depending on the context. See more on this Wave-Particle duality at Double slit experiment’s single photon interference
Photons also act as carriers of electromagnetic force while being exchanged between electrons on atoms’s orbitals.

Photon is born once an electron emits one in connection of returning from excited state to lower energy level. 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 excitation level. Photon got absorbed.
However, this excited state is not stable, electrons prefer staying on their normal energy level in the atom. Thus, after a very, very short while, electron returns back to normal 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. In the sun, in the torch’s bulb and even in the air. But in lesser amount there, as atoms in the air are so sparsely located.
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.

Quanta of the photon has to be exactly of correct size to make the electron excite. And vise versa. Energy quantum of the released photon depends exactly on the excitement level.
Different makes of atoms have different quantas for photon energies involved with this excitement.

Amount of photon’s energy decides the frequency of electromagnetic radiation the photon is representing. Thus, visible light sent by atoms of different materials is characteristic to material. E.g. atoms in blue paint has the electron orbitals such that electrons get excited exactly with the photons having wave length of 450nm or frequency of \(6.6*10^{14} Hz\).

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.

Actually, common understanding of the light being a ray is incorrect. At least by QED understanding.
Light from a torch consists of photons scattered to all directions from it. If you have a reflecting mirror on the way to your eye, it is not only in the ‘middle’ of the mirror, that the ‘beam’ gets reflected to your eye, as it appears to you. Each and every photon hitting the mirror is bound into arbitrary direction away from the mirror. Some of them towards your eye. Not only from the ‘middle’ of the mirror, but also from other surrounding points. The photons reflecting from the middle of the mirror have the shortest path from torch to your eye. The ones further away have longer distance to travel causing them to interfere with each other canceling themselves out. So it is only the ones with shortest path that really end up to your eye.