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## 12 March 2013

### The Physics Dork On Vacation: Dispersion in the Shower

While I don't see the term "dispersion" in the new AP Physics 1 and 2 curriculum framework, the phenomenon can be understood via the mechanism of refraction: different frequencies of light travel at (slightly) different speeds in the same material.  As a result, the different colors are bent at different angles.

The implication, then, is that any time sunlight passes through glass we should see a rainbow: red should bend at a different angle than violet.  So why don't we see rainbows everywhere inside a building?  Because virtually all windows contain parallel surfaces.  Sunlight disperses upon entering the glass; but then, upon leaving, the dispersion happens again.  With parallel surfaces and the same material (air) on both sides of the glass, the dispersion "cancels" and the light becomes white again.

Take a look at the top picture.  This is the shower in our spring break rental in St. Augustine.  The east exposure allows a lot of sunlight to enter, as you can see on the right side.

But look closely at the vertical stripes on the left-hand side of the picture.  They're rainbows!  The color fades from violet on the left to red on the right.

Burrito Girl, my wife and sidekick who does NOT teach physics, looks at the light pattern and says, "pretty."

I look at the pattern and say, "There's got to be a beveled window here somewhere."  Parallel panes of glass can't produce dispersion, but a prism-style window edge can: since the light hits the surfaces at different incident angles, the light dispersion doesn't happen in "reverse" as it does at the back end of a standard piece of glass.  Instead, the blue ray stays far away from the red ray, Pink Floyd-style.

Sure enough, a moment's ray tracing found the mirror in this lower picture -- you can see the beveled edge.

And Burrito Girl says, "You are an utter dork."  Yes, yes I am.

GCJ