Here’s another post based on the revisions I’m making for the second edition of my Singular Optics textbook! Caustics are a subject that I’ve sort of casually understood for ages but never well enough to explain it, but book work has finally made it possible.
Most of the history of optics concerns itself with designing lenses and mirrors that focus light to a point in order to make image-forming and correcting devices like cameras, eyeglasses, microscopes and telescopes. But light gets naturally focused all the time when it passes through irregularly shaped pieces of glass, reflects off of dented metal surfaces, or goes through water drops. The bright spots of light one gets look much more intricate than a simple spot of light, as a few examples below show.
The first image was a spot of light I saw on the ground at Gaffney Outlet Mall, created by light passing through some sort of decorative glass feature. The second image shows a coffee cup with three bright images inside, one for each light source illuminating the cup (the arrows show the direction the light is traveling). The third image shows spots on the side of a building in my neighborhood, created by light reflecting off of warped windows of a neighboring building.
These images are all very different, but closer inspection shows that they have similar features. They generally consist of a bright area surrounded by an even brighter line. These lines are the caustics. One can see that these bright lines often possess sharp cusp points.
Once you start recognizing these caustic features, you will see them everywhere. The other day I was getting out of my car and my open door reflected the setting sun onto the car next to mine. I had to stop and take a photo of how the small dings and dents in my car created caustic patterns.
But what are caustics, and how do we interpret images like the coffee cup caustic, i.e. what causes them? That’s what this post will be about.
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Skulls in the Stars 