Up until now, we’ve been discussing the effects of electrons moving up and down throughout the energetic bands of the electron orbitals, but we haven’t discussed the causes of what might make such an event occur. We’ve talked about the situations regarding the addition of doping elements, but these examples have stated in the “before and after” realm of what occurs. What about in real life, when not everything works in steady-state, unchanging conditions?
Well, summarizing about 400 years of scientific theory and discussion down into a more direct argument: in order for humans to observe what occurs, we must be able to see and record it. In order to see anything in this universe, our eyes must have light emitted from the object strike our eyes (or a camera’s) and register an image. In order for light to emit from the object we’re trying to observe, light must either be reflected from the target object (illuminated from another source), or the light must be generated and emitted directly from the target object. Either way, light is coming from the surface of the target object. Unfortunately, at the atomic level where we return to our example with the electron shell, light doesn’t interact in the same simplistic way.
Rather than simply “bounce off the atom,” like light appears to at our scale, every photon of light that strikes an atom is absorbed into one of the electrons, only to be re-emitted later as a different colored photon. The color of the photon being re-emitted back out into the world depends on three things:
- The material’s temperature.
- The material’s molecular structure and thus, its electron orbitals.
- The color or frequency of the light being first transmitted onto the material.
The temperature is first because it’s easiest to explain. Just as in an incandescent bulb, anything that heats up enough will start to glow (after it burns). That glowing is actually caused by photons (light particles) being emitted from the object into your eye, or camera, or whatever you’re using to look at the object. If a prism were used to split the light into its spectrum components, there would be only several bright “bands” that glowed, and the space in between would be darkness. These bands are the specific frequency or color of light that is being emitted from the material.
A great example of this effect is neon signs. We pump electric current through the material (neon gas) in order to “heat it up” on an atomic level. Heat itself is merely a form of vibrational energy in both the electrons and the atom’s nucleus. Eventually, the electron is vibrated to the point where it jumps from one orbital slot to another, casting off some of its excess energy in the process. This energy is released in the form of a photon (light).
So, when externally energized, or heated, an object eventually begins to emit photons on its own at certain colors. Since the slots only exist in very specific locations in the atom, the energy levels needed to jump from one slot to another also exist at very specific values. These very specific energy values are shed off by these photons, which are emitted at very specific colors. That is how these colors exactly correlate to the energy amount cast aside by the electrons moving from one orbital slot to another.
As the material heats up, or becomes more energetic in terms of vibrational internal energy, some of the electrons are able to jump to higher and higher orbitals, causing new bands to begin faintly appearing in the colors of light being emitted. The original bands remain, but become brighter or dimmer as the majority of the electrons in the material shift from one energy orbital to another. This is how the temperature of a material can directly affect the color of light being emitted from it.
Additionally, since the orbitals themselves and the energetic distance in between them is what determines the amount of energy being released, the electron orbitals in the material themselves are directly to blame for affecting the color of light being emitted from the material as well. This effect is literally what causes roses to be red, grass to be green, and the sky to be blue…it’s all based on the electron orbitals of the material we’re looking at. An object we see as green is reflecting or emitting primarily green photons, and little else.
That brings us to the final thing that could affect the color of the material we’re looking at, and the opposite side of the effects we’re interested in…which is what happens when a photon enters in and strikes an electron. Given that an energized electron drops from a higher orbit to a lower orbit and emits a photon, the exact opposite is also true. An electron struck by an incoming photon will energize the electron and possibly elevate it to a higher orbital state.
If the incoming photon is fairly weak in terms of energy or frequency, and doesn’t have what it takes to energize the electron it strikes to the next level, then it may be re-emitted just as it was, or stored for a bit by the electron. However, if the incoming photon has more energy than is necessary, the electron absorbs it all, jumping to the next orbital slot. After a short time has passed, the electron releases this extra energy in the form of a photon in one of the standard color bands. Any energy not released in the colored band is either emitted off as a lower-energy band as well, or stored until there is sufficient energy to do so.
In certain instances, when the incoming photon has an extremely large amount of energy, it can cause the electron to absorb so much energy that it’s blasted off of its parent atom entirely. This process of electrons being blasted off of their parent atoms by high-energy photons is called the Photoelectric Effect, and it only occurs with extremely high-energy light. Ultraviolet or UV rays typically have enough energy to blast electrons off of just about any material they encounter. X-rays, gamma rays, and cosmic rays all also have these same abilities.
The photoelectric effect focuses around electrons being blasted away from photons, but the process described before involving electrons being energized to a higher level is the general theory of “blackbody radiation,” one of the cornerstones to our understanding of the way the universe is put together. A very special section of the blackbody radiation theory is when the incoming photons have just enough energy to knock the electrons they strike into a higher energy orbit in the conduction zone. This special case situation is called the Photovoltaic Effect, and it differs in effect from the Photoelectric effect slightly by not having the electrons blown off the atom entirely.
Because the two different effects are caused by the exact same electron-photon energy exchange phenomenon, and really differ in terms of input energy and outcome effect, it’s easy to understand why the two effects can be easily confused. I’ve seen the reason for PV modules working attributed incorrectly to the photoelectric effect so many times…I feel it’s necessary to accurately note the difference. For a helpful reminder in case you get confused, it’s in the name: the Photovoltaic Effect is what makes Photovoltaic modules work.
<< Prev: The Diode and how it changed Electronics
Next: How PV Cells are designed to Collect Sunlight >>
