1905 was a great year for Einstein. A miraculous year, you might say. In fact it was so miraculous that it was coined Einstein’s Annus Mirabilis, Latin for ‘miracle year’. The reason why? He published not one, not two, not three, but four revolutionary papers that toppled the perspectives of matter, space and time during his generation. Few scientists are able to publish one revolutionary paper in their lifetime, much less four.
Today I will be focusing on just the first of his four papers. The article, titled ‘On a Heuristic Viewpoint Concerning the Production and Transformation of Light’ and published in the German physics journal Annalen der Physik on 9th June, built upon Max Planck’s idea of energy quanta.
During the 19th century, classical wave theory was the widely accepted model to describe the behaviour of light, primarily due to Young’s double slit experiment. However, the idea of wave-particle duality was also emerging, and there was slight controversy over whether light could behave as a particle also.
So pretend that we’re in the 1880s, and we’re starting with a simple metal plate, for example zinc. We know that metals consist of a lattice of positive metal ions, surrounded by a sea of delocalised electrons. We want to shine a beam of light onto the plate and check if any observations agree with the wave theory. If they do, then wave theory remains in acceptance. If they don’t, then we evidently have a problem with the theory.
This is the what the wave theory predicts:
- UV light: we should detect electrons being emitted from the surface of the metal. This makes sense, because the electrons in the metal are receiving enough energy from the beam of light in order to escape the attractive forces of the metal ions.
- Low-intensity UV light: we should still detect electrons being emitted, but more slowly, because energy from the wave is transferred at a slower rate.
- Visible light: we should still detect electrons being emitted, but also more slowly, because visible light carries less energy than UV light, and therefore energy is transferred at a slower rate.
For the wave theory enthusiasts, this is sadly not what happens:
- UV light: the observations agree.
- Low-intensity UV light: there is no delay in the time taken for electrons to be emitted. However, fewer electrons are emitted.
- Visible light: no electrons are emitted at all.
So Einstein racks his brain and puts out his new theory that would agree with the new observations. He proposes that when light is shone onto the plate, it is acting as a beam of particles (photons), where each particle has energy proportional the the frequency of the corresponding wave. UV has a higher frequency than visible light, so UV photons have more energy.
Einstein also proposes that the metal has a work function. This is the minimum amount of energy that the electron must gain in order to escape the surface of the metal.
The key to resolving this issue is that ONE photon interacts with only ONE electron, transferring ALL of its energy. If the energy of the photon is greater than the work function, the electron can escape. If the energy of the photon is less than the work function, even if it’s just under, the electron cannot escape. Any of the photon’s excess energy is converted into kinetic energy of the electron (i.e. the electron is emitted with greater speed).
And this effortlessly explains all of the observations:
- UV light: the energy of a UV photon is greater than the work function of zinc, so electrons are emitted.
- Low-intensity UV light: a lower intensity means a fewer number of photons. Therefore fewer electrons interact with a photon, and fewer electrons are emitted.
- Visible light: the energy of a visible photon is less than the work function of zinc, so no electrons are emitted.
This phenomenon of generating a current using light is now called the photoelectric effect, and began the quantum revolution in the early half of the 20th century. This is one of the major pieces of evidence for the ability of light to behave as particles.
In hindsight, it only required a bit of creative thinking that earned Einstein the Nobel Prize in Physics in 1921. And a ridiculous amount of mathematical calculations and physical knowledge, of course.