Snowflake specs

kw: observations, convolution, novelty glasses

Someone gave me a pair of "Snowflake Glasses" as a gag gift. Here is how they look. It is kind of intriguing. There are apparently many of these with different light effects.

 I could see that, whatever they did, it would be a diffraction phenomenon. The purple color hints that they use an Ozalid process to make these. That makes sense, because it is capable of very high resolution, and is used for cheap microfilm. I didn't try these while there were Christmas lights up, so when I did, I first looked at the sodium-mercury street light. A very pretty snowflake! You can see a dimmer one radiating from the reflection on the right.

I noticed a little red-green fringing, which I would expect from a diffraction effect. So I decided to try a more broad-spectrum light, as seen next.

This is the light on a neighbor's house. It shines at a side yard. The color fringing is more evident. Each wavelength is diffracted a different amount, with red moving furthest because its wavelength is the longest.

I just had to see what the pattern looked like under a microscope. That's next.

 Diffraction bends light both ways, so only three pattern orientations are needed to make the hexagonal star. This pattern is called a convolution. It is the mathematical inverse of the star pattern that we see when we look through it. The oval with "snow090" is an identifier. The ovals are spaced far apart, so they don't diffract enough light to be visible, and they do it at a very low angle.

The total pattern is made up of tiles that look like they are about 2"x2" in this image. They are small enough that several of them "fit" in the beam of light that enters the pupil of your eye. Thus, no matter what portion of the specs you look through, you see the intended snowflake.

I am not sure of the magnification here. I was using my inspection microscope at 20x, but this is cropped from the middle of a bigger image, so perhaps it is around 50x.

Just for the record, it would be possible to produce such glasses that select just a few light colors (red, green and blue, for example) and diffract them separately so that they all make the same size snowflake, and there are no color fringes. But it has to be done using a different method, and the cost would be much greater. These are intended for looking at Christmas lights, which are single-color.