Photoelectric Interactions

One of the strangest developments in modern physics came gradually, in the 19th century, as scientists learned more and more about the interactions between light and matter. In this post we’ll cover a few of the early experiments and their implications for both technology and our understanding of what light really is.

The first piece of the puzzle came when Becquerel found that shining a light on some materials caused a current to flow through the material. This is called the photovoltaic effect, because the photons incident on the material are causing a voltage difference which the current then follows. At the nanoscale, the photons are like packets of energy which can be absorbed by the many electrons in the material. In a semiconductor, some electrons can be moved this way from the valence band to the conduction band. Thus electrons that were previously immobile because they had no available energy states to jump to now have many states to choose from, and can use these to traverse the material! This effect is the basis of most solid state imaging devices, like those found in your digital camera (and trust me, we will delve further into that technology soon!).

But as it turns out, if you use photons that have a high enough energy, you can not only bump electrons out of their energy levels, you can cause them to leave the material entirely! This is called the photoelectric effect, and in some senses it seems like a natural extension of the photovoltaic effect: another consequence of light’s electromagnetic interaction with charged particles.

But actually, there is a very interesting scientific consequence of the experimental details of photoelectric effect. Imagine shining a light on a material, and observing the emitted electrons. You can change the light source in various ways, for example by changing the color or the brightness. Blue light has a shorter wavelength than red light, and thus more energy, but if you are only changing the color of the light you won’t see any difference in the electron output (unless you tune the energy low enough that no electrons are ejected, in which case you are back to the photovoltaic effect). But, if you change the intensity of the light, you find that brighter light causes more electrons to be ejected. This matters because at the time, light was thought of as a wave in space, an electromagnetic oscillation that could move through the world in certain ways. Waves with more energy were expected to liberate more electrons, just as higher energy waves in the ocean cause more erosion of the rocks and sand on the shore. But the experiment above disproves that idea, because it’s not the energy of each wave packet that matters but their overall number, which is the intensity of the light! So the photoelectric effect proves that light is quantized: while it has wave characteristics, it also has particle characteristics and breaks down into discrete packets.

The photoelectric effect is used in a few very sensitive photodetectors, but is not as common technologically as the photovoltaic effect. But there are a few weird consequences of the photoelectric effect, especially in space. Photons from the sun can excite electrons from space stations and shuttles, and since the electrons  then flow away and aren’t replenished, this can give the sun-facing side a positive charge. The photoelectric effect also causes dust on the moon to charge, to the point that some dust is electrostatically repelled far from the surface. So even though the moon has no significant gas atmosphere as we have on earth, it has clouds of charged dust that fluctuate in electromagnetic patterns across its face.

4 responses to “Photoelectric Interactions

  1. Pingback: The Electronic Eye: Charge-Coupled Devices | letstalkaboutscience

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