By Don Lincoln, Fermilab
Albert Einstein is known mostly for his theories of special and general relativity, but that’s not all he did. In fact, while Einstein did indeed win a Nobel Prize in physics, it wasn’t for his relativity work. It was for his work on the nature of light. What was it? Read on more to find out.
Black Bodies
In order to understand Einstein’s work, we need to take a very small step backward and talk about another mystery of the late 1800s, and that one deals with the color emitted by hot objects.
There are indeed lots of hot things. There are hot gasses and things like hot metals. The light emitted by each has a lot to do with their chemical makeup. But physicists of the time were interested in learning what color light hot objects emit when none of their chemistry matters. For that, they were interested in what are called black bodies. They are called so because they absorb all radiation that hits them. However, when they are hot enough, black bodies also emit light.
Ideal black bodies are somewhat hypothetical, but the easiest one to visualize is a steel foundry. In a steel foundry, huge heaters heat metal until it melts and eventually glows. Workers can look into the oven by opening a little door and seeing the glowing metal. Even light from the outside that goes through the door tends to bounce around and never come back out. The only light that comes out is the light from the hot steel.
The Wavelength of Light
According to the understanding of light from the 1800s, all wavelengths of light should have equal energy irrespective of whether they are long or short wavelengths. In reality, there simply are more short wavelengths. After all, the longest wavelength possible in an oven is a wavelength the size of the oven. Bigger ones won’t fit. But an infinite number of shorter and shorter wavelengths will. So, it stands to reason, that:
- With many more short wavelengths and
- All wavelengths carrying an equal amount of energy, the light coming out of the oven should mostly be short wavelengths.
Except that it wasn’t true.
This article comes directly from content in the video series The Evidence for Modern Physics: How We Know What We Know. Watch it now, on Wondrium.
Ultraviolet Catastrophe
In the visible spectrum, red wavelengths are longer and blue wavelengths are shorter. And, even shorter still is a form of light called ultraviolet. Ultraviolet is the kind of light that causes sunburns and skin cancers. According to the theories of the 1800s, light emitted from steel mills should be very blue and actually there should be lots of ultraviolet light. But that didn’t agree with the measurement. The energy was mostly found in the longer wavelengths, not the shorter ones. This conundrum was known in the late 1800s as the ultraviolet catastrophe.
Max Planck
The problem was solved in 1900 by the German physicist, Max Planck, who hypothesized that the energy held by light waves was not equal for all wavelengths. He hypothesized that the energy was proportional to the frequency, which is inversely proportional to wavelength. This means that short wavelengths have high frequency, and long wavelengths have low frequency. Planck asserted that shorter wavelength light carried more energy than long wavelength light.
Admittedly, this explained the ultraviolet catastrophe. If there was only a certain amount of energy in the oven and individual beams of short wavelength light carried more energy, then there had to be fewer examples of short wavelength light. And this is exactly what was observed. Planck took his conjecture, which was that the energy carried by a beam of light was equal to a constant times the wave’s frequency, applied it to the black body problem, and got perfect agreement with the data.
Now, Planck was thinking in wave terms, with light being emitted by vibrating atoms in the side of the oven. But Einstein, on the other hand, had other ideas. When it came to Heinrich Hertz’s mysterious problem of finding out that red, orange, and yellow light wouldn’t cause a spark, and blues, greens, and purples would, Einstein knew about this problem. It was he who combined it with Planck’s explanation for the ultraviolet catastrophe.
Einstein’s Photoelectric Effect
In 1905—which was the same year Einstein invented special relativity—he wrote a paper on what is now called the photoelectric effect. He hypothesized that a beam of light was actually a stream of particles of light. We now refer to them as particles of light photons. Although Einstein didn’t coin the term, it was made popular, in 1928, by Arthur Compton, although he didn’t coin the term either.
In any event, Einstein was able to explain Hertz’s observations in the following way. He proposed that the atoms in the electrodes held on to their electrons with a certain amount of force or, equivalently, energy. In order to make a spark, a photon of light would need to hit the atom and give it enough energy to knock it out. Since red, yellow, and orange photons had long wavelength they, therefore, had a low frequency.
Einstein and Planck’s Hypothesis
By Planck’s hypothesis, these photons didn’t have a lot of energy. In contrast, the green, blue and purple photons had short wavelength, high frequency, and therefore, enough energy to knock the electrons out of atoms. So, according to Einstein’s paper, everything could be explained, if light consisted of photons, which were individual particles. Furthermore, the energy of individual photons was proportional to their frequency.
Interestingly, frequency is a property of waves. Yet, Einstein said that light consisted of a series of particles with energy proportional to their frequency. That right there should already blow our mind!
Common Questions about Einstein’s Photoelectric Effect
Black bodies are called so because they absorb all radiation that hits them. However, when they are hot enough, black bodies also emit light.
Max Planck hypothesized that the energy held by light waves was not equal for all wavelengths.
Albert Einstein wrote a paper on what is now called the photoelectric effect. He hypothesized that a beam of light was actually a stream of particles of light.
