For millenniums, people have wondered what is light. Socrates hypothesized that light was made of streamers that were emitted from our eyes, enabling us to see the world around us. Throughout the middle ages, many scientists discovered that light had the characteristics of a wave, but were at a loss to explain what light really is. It wasn’t until the early twentieth century that the widely accepted Quantum Theory of Light was developed.
The Quantum Theory both explains what light is, and provides insight into the infinitesimal world of the atom. As well as providing a good starting point for anyone who wants to understand Quantum Mechanics.
Basically, light is a form of energy. Almost everything in our universe, falls into two categories, Energy or Matter. The Periodic Table of Elements is like a spectrum of matter, the first element Hydrogen is the lightest and simplest, with one proton, one neutron and one electron. The more electrons and neutrons and protons, the larger and heavier and denser matter becomes.
Now energy also occurs in a spectrum, somewhat the same as matter. Almost all forms of energy can be found in the Electromagnetic Spectrum. Visible light is a very small section of the Electromagnetic Spectrum, somewhere in the middle. Our eyes can only detect a few light waves, the colors of the rainbow, and a few others like white, gray, and brown. Some bees can see ultraviolet light, and a flower that appears just plain orange to us may be seen by bees, to have ultraviolet stripes across it. Even though we cannot see all the forms of light, we can sense and detect them in other ways. For instance, heat is a form of Infrared light, that we can sense with our skin. If something is hot like a wood stove, we can place our hands near it and feel the heat waves. We can also feel the warmth of the sun. Radio waves are another form of light we cannot see, but we can hear them after a radio converts them into sound. If we could see radio waves then we would see a radio tower light up when it is transmitting. If we could see microwaves then we would notice our cell phones and cell phone towers’ antennas light up when we placed a call.
But how is all this explained by the Quantum Theory? Well scientists discovered that electrons surround the center of an atom, and the electrons are arranged in levels or shells. It is these levels that are so important. Because electrons change levels. As an electron moves from one level to another, it either creates light or absorbs it. Now this happens because there are no in-between levels. This is the quantum part of the explanation. These levels are called quantum levels because they are the smallest known levels of energy and nothing can or does exist between these levels.
A good analogy is a stair case, to go up or down the stair case, you must take steps. These quantum levels are like steps. They must be whole steps, you cannot take half steps, or quarter steps, because only whole staircase steps exist. For an electron to move up these steps, it must absorb the exact amount of energy required go up one or more steps. And to go down a step it must release the exact amount of energy required to go down one or more steps, and this exact amount is most commonly a light wave or a light particle called a photon.
The difference between one quantum level and another is the same difference between the colors of light. For example lets just say an electron drops from level 5 to level 4 then it creates a violet colored photon. If it drops further to level 3 it will give off another color photon. And to go up the same level it must absorb a photon. The wavelengths of the different colors of light match the gaps between electron levels or shells.
The word Quantum has the same meaning as the word quantity, meaning when each electron changes level it requires a certain quantity of energy, and these specific quantities are photons or quanta of light.
What makes this discovery so interesting, is the fact that the quantum levels are a lower limit to the energy levels of our universe. The Electron is an incredibly small object and it is hard to imagine its size and photons are even smaller. But now we know that the small quantum energy levels that govern photons and electrons are the absolute foundation structure of energy, and no smaller energy levels exist.