Chalk Art 2024: Prism Recombination

Completed 2024-09-21. Available releases:

This is a chalk art painting I made on my square for the Forest Grove Chalk Art Festival in 2024.

There is a common misconception that if you stick two prisms back to back, the first one will split the light and the second one will put it back together. There are many diagrams—including on educational websites!—that show this falsehood. In actuality, you need a lens as well in the middle. (Please see my other artwork here for more explanation and discussion!)

I decided to draw a correct diagram (up to the limit of freehand sketching it, anyway) of what would happen if you don't have a lens. You can see that the light splits in the first prism, and then is split even further in the second prism. Additionally, some light reflects off of most interfaces due to Fresnel.

The best way to use chalk is to crush it up into a powder, mix with water, and apply like a paint. This covers a concrete sidewalk square more fully, and the result lasts longer. It took much longer than I anticipated to make this simple-looking drawing. I'm not sure what happened to the violet in the photo; it was there in the drawing.

See also my previous (2023) and next (2025) chalk art entries.

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