Prismatic Doors

Måns Hagberg (site) pictured this Swedish display of spectral colours emanating from the edge of a glass door. Image ©Måns Hagberg

Simple refraction through the 90° prism formed by the edge and side could not produce the colours at the suns position when the image was taken.

Closer investigation by Måns showed that the 10mm thick doors had a 45° bevel ~1mm wide on each edge.  Another clue was that when the door was rotated counter clockwise (looking from above the hinge) the coloured beam against intuition swung clockwise at first and then counterclockwise.

The keys to the colours are the small 45° bevels on the door edges. There are two ray paths that could form a spectrum across the floor. Path A - Rays enter the large door side and leave through the opposite side bevel. In Path B rays instead enter the nearside bevel and leave though the opposite door face.

In both cases the path is equivalent to passage through a 45° prism.

Hold a prism so that it projects a spectrum on a wall. Rotate it. The spectrum first moves one way and then another as the prism swings through the minimum deviation condition.

The two ray paths.

Look for prismatic spectra from glass doors, bus shelters, fish tanks and your cut-glass tableware.

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