The Bizarre Science of Light Production
If we are to understand the science of how LED’s produce light, we first have to understand something quite odd about electrons. Electrons can not do something that seems very simple – have “forbidden” quantities of energy.
To think of it as whole numbers, electrons could be at energy levels 1, 2, 3 but never, under any circumstances at 1.1, 2.4, or 3.000000001. The energy, in physics terms, is quantized (hence quantum mechanics).
So, why can’t an electron occupy energy levels in between these set quantities? Science has not fully answered that question but it has to do with the wave nature of electrons – there are certain places the wave aspect of electrons just do not want to be.
This bizarre reality about electrons creates two odd outcomes when electrons change energy levels. First, the electron “quantum tunnels” and instantaneously transports from one energy level to the next without ever occupying the space in between. Secondly, the electron emits or absorbs a photon of light.
Photons, in many ways, are the energy currency of the universe. If an electron were to absorb a photon it would go up an energy state equivalent to the energy of the photon. Electrons can also emit photons and go down an energy level equal to photons energy.
This process of producing light happens more than you likely realize. All matter produces some light. In fact, you right now are producing light. Truly. You are glowing. This light, however, is below the visible spectrum and leaves your body as infrared radiation.
Incandescents, Fluorescents, HID, and LEDs
For an incandescent bulb, the filament becomes so hot as electricity flows through it that it starts to visibly glow. This glow is caused by the molecules bumping into each other, causing electrons to go to higher energy states, and then relax down releasing photons. In the case of fluorescent lights, electrons jump from one mercury vapor molecule to another causing electron repulsion, excitation, and relaxation along the way, thus creating UV light that then gets converted into visible light by a white powder phosphor coating on the inside of the bulb. HID works similarly, although there are more types of metal within the bulb, some of which only become vaporized at high operating temperatures.
LEDs work differently, though. In a similar way to how light is produced when an electron goes from a high energy state to a low energy state within a single molecule, LEDs allow light to be produced when an electron passes from a high energy state in one atom to a lower energy state on a different molecule.
LEDs are made of two layers of electrically conductive crystals that have slightly different structures. One of these layers is referred to as the n-type; the ‘n’ denoting that it is negative and is the side that is the source of the electrons. The p-type side is the side that is “positive” and receives the electrons into lower energy “holes.” When electricity is passed through these two layers electrons drop in energy state and by doing so release photons of light.
Truly amazing, isn’t it?!