John Tinker writes: I believe that the tracks on a CD are concentric rings, which would make a CD something of a reflective Fresnel lens. Andy

But it's really not a very good lens as such, and probably not really responsible for the effect noted. The nearest equivalent optical surface for monochromatic light would be a concave cone. If the CD were a uniform disk, the rays from the center would focus at the CD, and the rays from the edge would focus at some distance further along. For different wavelengths the depth of the effective cone would vary, which would result in major chromatic abberation. The blue light from the disk edge would focus nearly twice as far from the disk as the red light would. If anyone knows the spiral spacing off-hand, I could tell you distance at which a given wavelength focuses. The real trick of a fresnel lens is the way in which the line spacing varies to provide a point focus for monochromatic light. Most fresnels also include refractive elements to correct for chromatic effects. Eric

That means for 5000 angstrom light the diffraction angle is 18 degrees. Light from the inner portion of the disk (2.25 cm) would focus at 6.8 cm Red (7000 angstroms) at 12 cm. That matches pretty well with what I see when I look into a disk with a strong light behind my back. The focusing reported is definitely a refective surface geometric effect and not a diffraction effect. Eric

John Tinker <jtinker(a)coin.org> wrote: Hi Because it was made that way. This could easily be the effect of putting the entire thing together. It is noted that the media part of the material is more to one side. Any difference in the coefficient of thermal expansion while pressing would tend to make a slight bowing. This would tend to make it consistently on one side. It could also be that the thickness of the coating was a little thicker towards the center, making a lens effect. If one only looked at warping, it would only take about 0.060 of a depression of the center to cause a 20 foot focal length. There are a number of reasons to discount the Fresnel effect. As was noted by another, the Fresnel effect would also split the colors to different focal lengths. Also, my original argument, if it was a Fresnel effect, the spacing is the same from both sides of the CD. This would indicate that the focal length would be the same from both sides. Dwight

A few months ago I was shown a couple of arrays of memory which used teensy Ruby Rods in place of the standard ferrite doughnuts. An array consisted of lots of the rods gridded out like core. The array was sandwitched between two clear, plastic, adhesive sheets. Why ruby rods over cores? Ruby sounds expensive. What were the benefits? How do you make teensie ruby rods and who did it? How common was this type of memory? Regards, Jeff -- ----------------------------------------------------------- Jeffrey S. Worley President Complete Computer Services, Inc. 30 Greenwood Rd. Asheville, NC 28803 828-277-5959 Visit our website at HTTP://www.Real-Techs.com THETechnoid(a)home.com -----------------------------------------------------------

Chuck McManis <cmcmanis(a)mcmanis.com> wrote: Hi Chuck I still like my manufacturing explanation better. Well, OK, yours is as good as mine but I had to defend my anyway ;) Dwight

At 09:24 PM 12/11/00 -0600, John Tinker wrote: How about the possibility that the disc is warped slightly when it's punched from a larger piece of plastic? - John