CD Spectroscope

A spectroscope is an optical instrument that determines the electromagnetic spectrum of light. This sounds complicated, but you all have seen one in action when you saw a rainbow. A rainbow shows all the spectral colors of the sun light and this is what an spectroscope (or a spectrograph to be precise, check Wikipedia for details) is all about.

There are a few interesting things to learn for kids in this project:

• Light is usually a mixture of light of different colors/wave-lengths
• Different light sources have different spectrums
  
• You can identify the type of light source by examining the spectrum
• With the spectrum you can also explain some properties of a type of light (also see below)
  

So now that you know what we want to build you might be glad to hear that it is really easy to do. All you need is:

• a CD (CD-ROM, not CD-Rs seems to be the best)
  
• a roughly cube-shaped cardboard box, large enough for the CD to fit in (a bit larger is better)
  
• some thin cardboard
• cardboard tube
  
• a bit of transparent, milky plastic film
• paper
• duct tape
  

How to build:

Cut a piece of paper to a size that it matches the size of the CD. Then cut out a small sector on the left side, like this:

Open the box and fix the CD and its paper mask to one side of the box. Don't put it at the center but with the visible part of the CD at the end of the area, like in the following image:

The image shows two more things you need to do. First make a square hole to the front side (opposing the side you have the CD attached to), about 2 inches/5 cm large. This is where the light will enter our spectroscope.

Then we also need a hole through which we can look at the CD. Make it on the right side. I used a card board tube that I fixed with duct tape to reduce the amount of light which can enter the box from there. Attach it in such a way that you can see the visible part of the CD:

Next we need to pay some attention to the hole the light enters. It should have the shape of a small slit. The smaller it is the more accurate our spectroscope will be but it also will be darker. I make it about 1 millimeter wide, which seems to be a good trade-off.

To make the slit, take a piece of thin cardboard and cut it to a size that it covers your square hole well. Then cut it into 2 equal pieces with a straight cut:

Then cover the hole with the 2 pieces, make sure you leave a vertical (assuming you have the box like in the image above) slit of 1mm and fix it with tape.

We want to have the CD evenly illuminated. For some light sources, e.g. direct sun, this does not work well as we will get a bright streak of light of the shape of our slit on the CD. To improve this we put a piece of plastic film onto our slit. Best work transparent but milky, non-colored films. They should look like this:

We only need a small piece large enough to cover our slit. Simply fix it with tape.

Great, now we are ready. Close the box, duct tape it so no light can enter anywhere and now you are ready to go. Hold the box so that light can enter through the slit and shine onto the CD. Look through the tube and the CD should show the spectrum of the light source.

Results 

Taking photographs is a bit tricky, but I tried my best with various light sources:

Sun light:

Light bulb:

Energy saving bulb:

Fluorescent lamp:

Green LED:

Red LED:

Blue LED:

White LED:

When you look at the spectrums (or is it spectra?), there are some interesting things to discover:

• The sun spectrum is quite uniform from red to blue. Only the blue parts are weaker. The reason for this is that these parts are partially filtered out by the atmosphere (and that's why the sky looks blue!)
• The spectrum of the light bulb is very similar to the sun. This makes us consider it as pleasant.
• The energy saving bulb and the fluorescent lamp have a different spectrum: the blue parts are more intense, they have gaps and peaks. They differ from the sun noticeably and that's why some people don't like this light.
  
• Not very surpisingly the green, red and blue LEDs mainly have their main color in their spectrum.
• The white LED has a uniform spectrum too going up to the violets.

Links

• There are quite a few CD spectroscope project descriptions on the net, some are linked in the Wikipedia article.
  
• A good one with many details and background info is here.
  
• The idea with the plastic film as diffuser is from there.