Photoelectric

When we think of electricity we typically imagine a stream of electrons flowing through some conductor carrying the electric current. It is an innocent enough way to picture this phenomenon. It is not unlike the red blood cells carrying oxygen to our lungs through our arteries. Here, the red blood cells represent the electrons while the oxygen represents the charge of the electron. The arteries are the conductors or wires.

But what gets these electrons to move in the first place? In most, if not all, cases, the root cause for anything to happen is that there must be a gradient, a difference in state or level. For water to flow, there must be a height difference or a pressure difference, for example. For blood flow there must be a pressure difference between the heart and the rest of the body. Similarly, in an electric circuit, for electrons to flow there must be a difference in voltage between the battery and the rest of the circuit. The battery, thus, acts like a pump, not unlike the heart, creating this gradient, this difference necessary for the flow to take place. Blood will flow as long as the heart keeps pumping. As long as the battery keeps pumping, electricity will flow in the circuit. Like the heart, the battery can’t keep on pumping forever; it will run out of steam, so to speak, at some point, spelling the end of the journey.

How does the battery maintain this gradient? It stores more of the electrons on one side than on the other. The side where there are more electrons is generally termed the negative while the other is the positive terminal. When a battery has run flat what’s happened is that this gradient of electrons has levelled up. And so when we recharge the battery, we are simply reshuffling the electrons inside it to recreate this difference: grabbing electrons from one end and dumping them on the other. Only then can the battery start pumping again.

But we don’t always need a battery to get electrons to move and thus have electricity. It’s been observed that electrons can be knocked off a metal plate by shining a light on it and collecting them onto another plate. And if we can keep this going, if we can keep transferring electrons from one plate onto another then we’re creating a difference in the accumulation of electrons on these two plates. On one end there will be more electrons than the other. And by joining those two plate together via some wire, we’re completing the circuit and allowing this gradient to exist. With the gradient in place, we can expect the electrons to flow back from one plate to another, exhibiting this thing we call electricity. At some point, the electrons will level up. Just like a tank of water flowing onto another until some common level is reached. We can restart this process all over again by shining light onto one plate. A simple enough experiment to demonstrate electricity and, more interestingly, how without a conventional battery electricity is able to flow as a consequence of shining a beam of light onto some metal.

The magic of photons

The magic of photons

This phenomenon of knocking electrons off a metal surface with light is called the photoelectric effect. This has been observed many a times but an explanation couldn’t be formulated until a certain Albert Einstein came along. Heinrich Hertz experimented with electromagnetism and, in particular, the emission of such electromagnetic radiation. The unit of frequency (symbolised by Hz) is named after him. Radio waves, for instance, have a range of frequencies from about 3 Hz to 300 million Hz. Hertz was the first one to observe the photoelectric effect, in 1887, but could not account for the seemingly strange occurrence. You see, the idea that a beam of light could influence electrons didn’t seem to tally with the way light was thought to behave. Light, like other electromagnetic radiation, was just that: a radiation; a wave. How can a wave influence a particle (in this case, an electron) and cause it to be emitted from a metal?

It wasn’t until the concept of quantised energy came about that Einstein found the answer to this conundrum. In 1900, the great German physicist Max Planck, postulated that energy can exist in small, discrete packets. This revolutionary idea was what gave birth to quantum physics. In 1905, Einstein took this concept a step further by suggesting that not only does energy exist in small packets but, more interestingly, it can be transmitted in such form. Those little bundles of energy being transferred from one place to another was known as light-quantum or wave-packets of energy. It wasn’t until the 1920s that the name ‘photon’ was coined for these energy bundles.

Now, why was it important to think of light as being this quantised unit of electromagnetic energy when explaining the photoelectric effect? Prior to Planck or Einstein, the idea was that the electrons being ejected from the metal surface would do so with more energy (kinetic energy in this case) if the intensity of the light beamed onto the metal was increased. This seemed pretty straightforward enough. But that intuitive way of thinking about this problem didn’t tally with experiment. The energy of the emitted electrons didn’t depend on the intensity of the light but rather on the frequency. Yet the intensity did have an effect after all. The intensity controlled the number of electrons being emitted. The higher the intensity the more the number of electrons. However, if the frequency of the beam of light wasn’t enough then no matter its intensity, it would fail at knocking these electrons off the surface. These observations could be repeated over and over again but no explanation, based on the then prevailing theory of electromagnetic radiation, could be found.

Enter Einstein. He had the brilliant idea of taking what Planck suggested about quantised light and applying that to the photoelectric effect. What Planck had shown is that the energy of an electromagnetic radiation depended on its frequency. What Einstein then did is propose that the energy being carried by the electromagnetic radiation was transferred onto the electrons in the metal. If the energy was sufficient enough, it would dislodge the electrons from the metal surface. If not, no electrons would be emitted no matter how intense the radiation was. But, once the correct radiation with the correct frequency was beamed onto the metal, the number of electrons emitted could be controlled by the intensity of the radiation. It was as simply as that! As with any other genius ideas, it is not the complexity of the problem that is impressive but, rather, the simplicity of the solution.

The photoelectric effect as described by Einstein in 1905 was finally properly tested in 1914 and, in 1921, Einstein was awarded the Nobel Prize in physics for his predictions about that phenomenon. The photoelectric effect consolidated the idea that light can exhibit both wave and particle characteristics. (Please see Double for more about this dual nature.)

There are several applications of the photoelectric effect, namely in image sensors and night-vision devices. But the greatest impact, in my opinion, of this interesting phenomenon is on the development of the branch of physics we call Quantum Physics. Without Quantum Physics, none of the amazing pieces of technology we have today would have been possible. To understand why, see my blog on Quantum.

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