Hello, and welcome back to MPC! Last week, we discussed Einstein’s explanation for the photoelectric effect. Specifically, we discussed how the particle (photon) model of light is able to describe this strange phenomenon. This week, we will discuss another perspective on the nature of light: wave-particle duality!

Recall our discussion of the double-slit experiment from a few weeks ago. In that discussion, we determined that light has to be a wave. Why? Under the conditions of the double-slit experiment, light will interfere constructively in some places and destructively in others, forming a peculiar pattern:

Figure 1: The double-slit experiment (with light)

(image source: http://www.physics.louisville.edu/cldavis/phys299/notes/lo_interference_fig3_inf.gif)

Because we cannot describe the interference of light using particles, we determined that light must be a wave!

However, last week, we analyzed the photoelectric effect. In that post, we saw that the photoelectric effect cannot be described by the wave model of light — it can only be described by the particle (photon) model of light.

Figure 2: The photoelectric effect

Something strange is going on! A few weeks ago we “proved” that light is a wave, yet just last week we “proved” that light is a particle. What is light: a wave or a particle?

Here comes an important question: does it have to be either? Or should I say, does it have to be one or the other? As humans, we tend to like thinking about particles and waves. Why? Because they are easy to visualize. For instance, although many of the waves we have been dealing with are extremely small, there is no doubt that they have properties that are similar to ocean waves:

Figure 3: The waves we have been discussing are similar to ocean waves

Likewise, although the particles we have been discussing are extremely small, they can easily be modeled by, say, tennis balls:

Figure 4: The particles we have been discussing are similar to tennis balls

In other words, particles and waves have macroscale, real-world analogs which makes them very easy to visualize/analyze. However, just because particles and waves are simple models to visualize (and the models that we have the most experience with) does not mean that they are the only models.

Let’s introduce a new model. This model is strange in the sense that it sometimes behaves like a wave and other times it behaves like a particle. Of course, this model does not have a macroscale, “real-world,” “everyday” equivalent. Nonetheless, we can illustrate it as a strange wave-particle mixture:

Figure 5: The wave-particle model

**Note: This is not an “official” way of representing the wave-particle model (and “wave-particles” is not an “official” term). This drawing is simply meant to illustrate an idea on a conceptual level. We will learn about better ways of illustrating the wave-particle model in future posts.**

Could this model describe light? Perhaps it could! As I said previously, with this model, light sometimes behaves like a wave and other times behaves like a particle. What if this “wave-particle” light, under the conditions of the double-slit experiment, behaves like a wave? That would mean that light would be able to interfere with itself during the double-slit experiment and create a pattern like the one seen in Figure 1!

Similarly, what if this wave-particle light, under the conditions of the photoelectric effect, behaves like a particle? That would mean that light (“photons“) would be able to knock electrons off of the metal in the photoelectric effect (in the manner that has been experimentally confirmed)! This wave-particle description describes everything perfectly!

Satisfying right? Alright, perhaps not. Although this wave-particle model of light, or quantum model of light, is able to describe our experiments, it seems a little strange — how is it even possible for something to act like a wave sometimes and a particle at other times? This is a common theme with quantum mechanics — at first, most of quantum mechanics seems to contradict our everyday experiences. However, as we dig deeper into this subject, we will grow more accustom to its oddities and uncover explanations for these “contradictions.”

That is it for this week. This is just the beginning of our journey through quantum mechanics. Things are about to get truly amazing! I am excited, and I hope you are too! Next week, we will see if this wave-particle model can describe anything other than light. See you then!

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For more information (and a head start on future posts), be sure to check out these resources: http://maggiesscienceconnection.weebly.com/dual-nature-of-light.html, https://www.youtube.com/watch?v=Q_h4IoPJXZw, http://passyworldofmathematics.com/Images/pwmImagesEight/SurfWavesFive550x387JPG.jpg

(featured image: https://jonettecrowley.files.wordpress.com/2015/03/wpid-day-night-jpg.jpeg)