Skulls in the Stars

In the first part of my series on ‘What is a wave?’, I attempted to give a broad definition of a wave, so that we can identify them when we see them. In this part, I will address two of the most important behaviors of waves: interference and diffraction. Interference may be loosely described as the interaction of a wave with itself, or a wave with another wave, while diffraction may be loosely described as the interaction of a wave with other objects.

We will discuss interference in this post, and consider again the wave on a string discussed in part I of this post. A pair of waves are sent down the string to a fixed end, where they are reflected and return to their point of origin. What happens when the waves pass each other? An animation of such an event is displayed below:

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As I’ve mentioned a number of times before, my optics specialty is physical optics, which is the study of the wave properties of light. In order to understand those wave properties, however, it is important to understand what a ‘wave’ is and what it can do. This article is an attempt to answer these questions in a non-technical way for the layperson.

This is not as easy to do as one might think. Most of us are aware of numerous wave phenomena: waves on a string (such as a guitar string), water waves, sound waves (acoustical waves), seismic waves (earthquakes), ‘The Wave’ at football games. There are also many less familiar examples: light waves, particle waves (quantum mechanics), gravitational waves (caused by collapsing stars, for instance). It is quite difficult, however, to explain what these phenomena have in common. Furthermore, as we will see, any definition of a ‘wave’ that we come up with will have exceptions. I suspect most physicists would give a definition of a wave that’s similar to the Potter Stewart definition of hard-core pornography: “I know it when I see it.” We will do the best we can, though, and note those exceptions as they arise.

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