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Glass Bead Bow on Tarmac

When passing a cross-walk, a colourful bow is not what one expects to see there. So Jordan Stabile was very surprised when discovering the colourful bow on the tarmac in Somerville, Massachusetts, USA.

The answer to this problem is a rainbow, but with the difference that in this case the light is not reflected by drops of water, but by transparent spherical glass pellets. These have a diameter of about 0,25 millimetres and are used for sandblasting especially at cross-walks. As glass refracts light in a bigger angle than water does, the bow is smaller than a normal rainbow, with a diameter of only 20°.

A glass bead bow can easily be reproduced by placing a lamp over a sheet of black paper, on which glass beads (available for example in building supplies stores) have been spread before. Then a colourful bow (photo) appears on the paper. As the distance to our two eyes is very small in this case, the bow can also seem to float tridimensionally over the paper.


Tertiary Glass-Sphere Bow

I have discovered a spectral reflection phenomenon inside a transparent plexiglass-sphere. The phenomenon, of which I am almost sure it is NOT the equivalent of the Primary or Secondary Rainbow, is in fact the equivalent of the Tertiary Rainbow, visible as a bright illuminating spectral colored ring all along the limb of the sphere. To see this ring, one should look “from behind” the sphere, toward the sun, with the sun “in front” of it (appearing exactly “in the centre” of the sphere).

The photo show the sphere with appearance of the red component of the spectrum. The distance of the observing eye (or camera’s lens) to the sphere is VERY important, because the focal point of the ring is not a point, it’s a spectral colored line (red at the far end, blue at the near end).

As far as I know, no one has ever observed or photographed the ring-like appearance of, what I call, the Tertiary Glass-sphere Bow (which has a focal point or “line”, behind the globe!).

Author: Danny Caes, Ghent-Belgium

Glass bead bow

While at work I was on break time I took some blast sand out of the sand blaster and scattered it on the ground and I happened to look down and saw a bright bow around the head of my shadow. The bow is like a rainbow but the higher refractive index of glass made a bow with a radius of 28 degrees rather than 42 for a water drop rainbow. Look for these glass bead bows on reflective signs and freshly painted black top.

Posted by Michael Ellestad