CD's as parabolic mirrors?

From: Eric J. Korpela <>
Date: Mon Dec 11 15:58:02 2000

> > I believe that the tracks on a CD are concentric rings,
> > which would make a CD something of a reflective Fresnel lens.
> >
> It's a single spiral groove, but the macroscopic optical effects are
> probably similar.

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.

Received on Mon Dec 11 2000 - 15:58:02 GMT

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