CD diffraction oddity

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From: MC elover (mcelover@yahoo.com)
Date: Thu Mar 16 2000 - 09:08:04 PST 

Message-ID: <20000316170804.16650.qmail@web904.mail.yahoo.com>
Date: Thu, 16 Mar 2000 09:08:04 -0800 (PST)
From: MC elover <mcelover@yahoo.com>
Subject: CD diffraction oddity

hello

My kids were using laser pointers to find the
seperation of the "grooves" on a CD. They reflected
the light off of the CD (using sophisticated
exploratorium optical stands--binder clips!) and
measured the distance between the dots on a wall a few
meters away. We got great results, but the kids
noticed something strange: some of the dots were made
up of smaller dots! We thought for a minute and then
reasoned that the small dots could be interference
(sorry, paul d) from the actual pits on the CD. What
do ya'll think? Does anyone know the size of the pits
on a CD?
        Also, if a CD can store 650 MB of info which
translates into 74 mins of music, is it possible to
calculate the bytes per second (and consequently the
bits per second) simply by dividing? Then perhaps I
could use the known? value of 625 lines/mm to find the
length of the "grove" and then find the number of
seconds per mm of groove length. (looks like i'll have
to use some calculus to integrate the increasing
radius "circles" of lines (wait a minute, that will
get me an area wont it?)) finally i could multiply
the seconds/mm by the bits/second and i would get
bits/mm. Someone tell me to stop before i actually
try this!

--eric

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