Photons Are Particles

Over at Dot Physics, Rhett is taking another whack at photons. If you recall, the last time he did this wasn’t too successful, and this round fares no better:

So back to the photon. In my original post I made the claim that the photoelectric effect is not a great experiment to show photons. Maybe that is not how it came off, but that is what I meant. The photoelectric effect can be explained quite well with the classical electromagnetic waves model and a quantum nature of matter. Of course there is a quantum nature to light as well.

I think the biggest problem with the photon is that the manner it is introduced encourages students to think of it as an actual particle. One thing about particles is that they are localized. I am pretty sure that even quantized light (the real photon) is not confined to a set space.

I’m no field theorist, but I can’t figure out what that last sentence is supposed to mean. I think it’s probably intended to be a reference to the idea of a photon as an excitation of a particular mode of the electromagnetic field, with those modes necessarily being extended over some region of space. But if you want to say that that disqualifies photons from being “actual particles,” then an electron isn’t an “actual particle,” either– the same field-excitation language works for electrons, as well as photons.

The fact is, photons are particles, in every way that matters.

When you detect photons with a CCD camera, they show up as discrete localized spots on the CCD. When you put photons in a cavity (two mirrors facing one another), you can see discrete, localized excitations of that cavity– you can “count” the number of photons in the cavity. When you look at the absorption and emission of light by single atoms, you see discrete jumps in the energy of both the light field and the atom itself.

Any test you can devise for particle-like properties of light, the photon will pass.

Now, it’s true that you can explain the photoelectric effect in semi-classical terms, with light as a continuous classical electromagnetic wave. Given that, why do we teach about photons and the photoelectric effect in introductory modern physics classes? Three reasons:

1) The semi-classical explanation of the photoelectric effect is much more complicated than the photon model. It requires the idea of energy quantization of electrons in a solid (basically, you treat it as an excitation from a discrete quantum state in the solid to a continuum of free-particle states outside the metal), and some understanding of the coupling between light and atoms. These concepts are well beyond students in introductory classes, and the math involved (the Fermi “Golden Rule” for transition probabilities) is way out of reach for those students.

2) The photoelectric effect gets at the idea of energy quantization for light, and the relationship between energy and frequency. This is essential when talking about atomic states– you need to know that light carries energy proportional to the frequency to explain the observed spectra of atoms, whether that’s via the Bohr model or a full quantum treatment of hydrogen. Spectroscopy is the best tool we have for studying the internal structure of atoms, and understanding spectroscopy requires understanding the energy quantization of light.

3) It’s a pedagogically useful experiment. Not only can you do the photoelectric effect in lab, but you can easily use it to set up two-equations-two-unknowns problems, which are great practice for students at the intro level. You also use the same photon concept to understand the Compton effect (another experiment that has a very difficult semi-classical explanation, and a really simple photon-based explanation), which makes another excellent sophomore-level lab.

And the idea of quantized light is all over modern physics. The notion of photons as particles with discrete energy and momentum turns up in atomic physics, condensed matter physics, nuclear physics, particle physics– just about any active field of physics research will use the photon concept at some level.

Photons are particles, in every meaningful sense of the word, and students need to know that photons are particles. You cannot claim to have even the most rudimentary grasp of modern physics without knowing that photons are particles.