CD's as parabolic mirrors?

From: Dwight Elvey <elvey_at_hal.com>
Date: Mon Dec 11 17:02:28 2000

John Tinker <jtinker_at_coin.org> wrote:
>
>
> Eric Smith wrote:
>
> > > 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 why would it appear concave from the label side, and convex from the
> data side?
>

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
Received on Mon Dec 11 2000 - 17:02:28 GMT