Re: Why do scratches in glass alway seem to form circles around light-sources?

Date: Tue Nov 23 21:00:15 1999
Posted By: William Beaty, Electrical Engineer / Physics explainer / K-6 science textbook content provider
Area of science: Physics
ID: 942923651.Ph

Message:

Hi Valerie.
Good question! I remember wondering about this as a kid, only I was thinking about the tree branches at night. In the winter, the ice-coated tree branches near a streetlight formed a "halo" of circles around the light. The branches were all over the place, yet they created a circular pattern around the light at night. As I walked along, the circle followed me, even though the branches did not.

If you look carefully at the plastic, you'll see that the glowing "circle" is actually made up of short pieces of straight scratches. All of these short straight pieces form a circle, sort of like this:

                     _          
               /                
                  - _   \       
         |      /           |   
              |     *   |            (the light is in the center)
                 \ -            
                    _    /      
             \    _             
                               /

Each scratch is really far longer than these short bright parts shown above. If you move your head along, the short bright pieces seem to move along. That gives us a hint as to what's happening. We are seeing each bright highlight in one spot on each scratch, while the rest of the scratch remains dark.

We see this:



             *            <----the light

                     /        <---- the "highlight scratch"

Even though this the true situation is this:


                        /
                       /
                      /
             *       /
                    /
                   /        <---- long scratch
                  /
                 /
                /

The little "highlights" appear because the scratches are polished and shiney. They act like little mirrors (well, more like long silver bars.) The "highlight" on the scratch is actually a distorted image of the light source! The highlights follow the movements of your head in the same way that the light source does. The rest of the scratch remains invisible. If you move your head back and forth, the distant light source seems to stay in the same place, and all the little highlights try to do the same, even though the scratches are apparently sweeping sideways. The distant light source seems to be surrounded by little reflections, and the reflections form a circular halo.

All of this can only work if the scratches are shiney. And when I noticed this same phenomenon in the trees, it only worked because the branches were coated with wet shiney ice, and the tree was dark except for the "highlight" reflections of the streetlight on the ice.

There's another place where you'll see something similar. If you're walking out in a garden and a long piece of spiderweb is stretched across your path, you might only see the "highlight" of the sunshine reflected from the invisible strand. That "highlight" apparently moves along as you move your head. It is an image of the sun being reflected by the shiney strand. If you go and find an entire complicated spiderweb, try viewing a distant light source through it. You'll see that the same "circle" appears, just as with the polished scratches.

Here's something that these scratch-reflections can be used for:

SCRATCH-HOLOGRAMS DRAWN BY HAND
http://www.amasci.com/amate ur/holo1.html

By placing the scratches in just the right positions on the plastic, we can use the little "highlights" to draw pictures one dot at a time. What's cool is, these pictures can be drawn in three dimensions, so the picture hangs in space like a hologram. It's also possible to draw a "picture" of your hand by using a dirty paper towel to "polish" the plastic in a circular motion. See the above holograms article for pictures of how I "polished" my car hood with a gritty piece of paper and left glowing images of my hand behind.

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