CD's as parabolic mirrors?

From: Eric J. Korpela <>
Date: Mon Dec 11 19:17:26 2000

> > If anyone knows the spiral spacing off-hand, I could tell you distance at
> > which a given wavelength focuses.
> The specified track pitch is 1.6 +- 0.1 micron.

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
from the disk, light from the outer portion (6 cm) would focus at 18.2 cm
from the disk. Voilet (4000 angstrom) from the edges focuses at 23 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.

Received on Mon Dec 11 2000 - 19:17:26 GMT

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