Spectra on a CD
                               by
                         Richard Hill*

Few can look at a spectrum for the first time without and exclamation of surprise and wonder. Nowhere in
nature are colors so pure and vivid. The making of a spectrum has to most laypersons and school children
always seemed a sacred art. Many know it is a means to study everything from the atom to galaxies but how
this is accomplished  is as mysterious as these objects themselves. 

For help with terms used in this paper, see the glossary at the end. 

A simple spectroscope consists of a slit, some method of breaking the light into its array of colors, and some
means of viewing that. Most classroom spectroscopes are a complex system that has a small telescope
focusing the light onto a precision machined slit, frequently one that can change its aperture. This is followed
by a one or two lenses that send a collimated shaft of that light to a prism or grating either of which can be
rather costly. Finally another telescope is needed for viewing the spectrum itself. Sound complicated? It is and
the cost for such instruments is often beyond the budget of many schools, and may even be as much as the
entire budget for science materials in elementary or primary schools. But it is not necessary to get this
complicated.  Now, with the advent of the CD anyone can make a device that will allow them to not only view
a spectrum, but do some crude analysis of bright light sources.  


MATERIALS...

-a box 6x12x4 inches or larger
-one old but good condition CDROM disk
-head alum. foil or opaque thin plastic that can be cut with scissors (heavy black
  construction paper can be substituted too)
-masking tape or better an opaque black tape.


CONSTRUCTION...

The most difficult thing to obtain will be CDs. But they are not only used for music and computer games. Many
companies, particularly internet service providers, send them out free to prospective customers. Thousands
of these disks are tossed out as useless garbage daily. A teacher or hobbyist need only collect these discards
to be on the way to enjoying the rudiments of spectroscopy. 

Minimal materials needed are a box 6x12x4 inches or larger, some black or very opaque construction paper
or black tape, and of course, the CD. Setting the box on end, cut a rectangular hole about one to two inches
long and half an inch wide near one corner centered on one of the 4 inch sides.  On the side of the box draw
a line from the center of that hole to the opposite side and then another line at 50 degrees going out the
perpendicular side. This will tell you where to cut another hole for viewing. Cut two pieces of the construction
paper and place them (glue or tape) over the first hole such that the form a slit about one millimeter wide.  In
the middle of that side, cut out several more pieces and arrange them so the slit is only about half an inch long.
Place the CD in the box laying flat on the side opposite the slit such that the side of the CD nearest the
viewport is right where the vertex of the drawn angle is (see fig. 1). Make sure the recorded or colorful side with
no printing is facing up. 



                                                  (c) Richard E. Hill 1998

USE...

Most schools and even homes have one or more flourescent lights. Place the spectroscope under one such
that the line drawn from the slit to the CD is pointing at the light. By looking in the viewport you will see a thin
shaft of light striking the CD and where it does you should see a spectrum vertically across the CD.  By
moving your head up and down along the viewport hole you should be able to see the entire spectrum from red,
through yellow, green, and blue to violet. Also notice the bright horizontal lines crossing the spectrum 
perpendicularly in specific colors. These are emission lines. The most prominent ones are in the yellow, green,
blue and if your lucky, in the violet. Note these on a drawing writing down their colors. 

Now go to an ordinary incandescent light bulb. Notice that the colors are there but none of the emission lines.
This is called a continuous spectrum where the colors are continuous and unbroken by any dark or bright lines. 

If it is daytime you can take the device outside and let skylight near the sun pass through the slit into the
closed darkened box. This spectrum should be brilliant and you may see extremely thin dark horizontal lines
crossing the spectrum. These are absorption lines.  They will almost seem to not be there or seem to float
above the colored spectrum. Solar absorption lines are very difficult to see and may well not be seen at all.

At night get close to  different colored street lights. What kind of spectra do they produce? If they are emission
spectra, do the lines appear in the same color regions as the  fluorescent light?  What kind of spectrum does
it look most like? What can you conclude from that? Note also which colors you can see. You will have a new
understanding why some lights are blue-white and others that pinkish color
(like the sodium parking lights. 

Improvements to the sharpness and resolution of lines in the spectra can be had by getting a box that allows
the slit to be further from the CD while your eye can remain the same distance. If a box is too narrow, a CD
can be carefully trimmed in width using scissors with a good edge.  Likewise, if there are limited CDs available,
some or all could be cut in quarters. The hub or center of the CD should be away from the viewport for best
viewing. Be careful not to scratch up the surface of the CD as these fine scratches will detract from the
performance of the device. 

GLOSSARY...

Absorption lines - The dark lines that perpendicularly cross an absorption spectrum. 

Absorption spectra - Spectra where the lines are black on a bright spectral background (continuum)

Continuum - The bright background made by a prism or grating when looking at a normal incandescent
     light bulb. This is often the background on which spectral dark or bright lines are laid when
     looking at things like the Sun or Moon (dark lines) or a fluorescent light (bright lines).

Continuous spectra - Spectra where there are no lines, only a smooth gradation through the colors
     from red through violet. Most easily seen in the spectrum of an incandescent light bulb.

Emission lines - The bright lines that perpendicularly cross an emission spectrum.

Emission spectra - Spectra where lines are bright against a dark or weak background of spectral colors.

Grating - A piece of glass or plastic on which lines of a particular shape are scratched or laid on such 
     that they break light up into the spectrum.
                                                                                              (c) Richard E. Hill 1998

Prism - A large piece of glass in one of a number of geometrical shapes that can, in certain
     orientations, break up light into a spectrum.

Spectra - The rainbow of colors produced by a prism or grating (a CDROM acts like a grating). One is
     called a spectrum, many are spectra. There are no such things as spectras, and spectrums. 



*This paper is copyright (c) Richard E. Hill 1998