Re: CD diffraction

[Dan Burns <kilroi@LGHS.NET>]

I am puzzled by the lack of a colored reflection. We do a lab where the
students calculate the pit spacing on the CD using the distance between
the white reflection and the colored diffracted reflection. It ties in
nicely with Feynman's book "QED". If the CD is used to reflect sunlight
you will see a central white reflection surrounded by a circular
spectrum. The spectrum is dimmer, maybe you just overlooked it. See the
Feb. 93 "The Physics Teacher" for more info.

Dan Burns
Los Gatos High School

Ludwik Kowalski wrote:

> This was after the diffraction grating lab today. A student
> wanted to see the effect of a CD on light. I turned the
> laser on and we at once saw the central strongly reflected
> beam, then two less strong beams at about 30 away, and
> another par at about 60 degrees etc. I was happy to say
> "this is m=0, these are |m|=1, 2, etc.".
>
> Then the beam of white light was reflected. What we saw
> was a band of white light. We expected the band to show
> colors, as from the diffraction grating. But there were no
> colors. Why? Details on how to disperse colors with a CD
> will be appreciated.
>
> Ludwik Kowalski