Mini Spectroscopes

Most of what we know about the Universe comes from looking out in to space, and analysing the light (and other electromagnetic radiation) that reaches us.  Spectroscopy is one of the tools that we can use, and it allows us to determine what chemical elements things are made of.

Colour Spectrum

First a bit of background.  A Blackbody emits electromagnetic radiation (e.g. light) based only on its temperature.  An actual perfect Blackbody doesn’t exist, but the underlying equations are a good match for what we see in the real-world.  The diagram below shows the intensity and wavelength of emissions for different temperatures (shown in Kelvin).  Cold things (e.g. 300K) have very low intensity emissions off into the infra-red.  Very hot things (e.g. 10,000K) have very high intensity emissions off into the ultra-violet.

Light intensities for Blackbodies with different temperatures (in Kelvin). CC-BY-SA-3.0 Sch.

Our Sun has a surface temperature of around 5777K – which places the peak of the curve near the middle of the visible spectrum.  This isn’t a coincidence; our eyes have evolved in the presence of this light, so it makes sense that this is the range of frequencies that we are optimised for.  Of course, in the light that comes from the Sun these colours are combined to make what we see as white light, but we are also used to seeing the colours separated in rainbows (and soon in spectroscopes).

When light interacts with matter, both the light and matter are changed.  The matter absorbs some frequencies of the light, leaving gaps in the spectrum (an absorption spectrum).  The matter can then re-emit the same frequencies in different directions, creating a very broken spectrum (an emission spectrum).

A continuous spectrum from a light source on the left, leading to absorption and emission spectrums.

Shown below are some different emission spectra for familiar elements.

Some example emission spectra found on Wikimedia Commons (tagged public domain)

Note that the spectrum emitted by the Sun is not actually a continuous spectrum.  Because the light generated in the core passes through the outer layers, some wavelengths are absorbed, and an emission spectrum is produced.  The image below shows how an analysis of the gaps reveals the different elements that the light has passed through.  Also note that the presence of some elements, such as Oxygen (O2) are caused by Earth’s atmosphere – since the observation was made on Earth.

Absorption spectrum for the Sun. CC-BY-SA-3.0 Anonymous.

Through these techniques it is possible to determine the chemical composition of stars, and even the atmospheres of planets orbiting those stars.  The simple spectroscopes shown below won’t provide that level of precision; but you will be able to see the physics in action.

Cardboard Box Spectroscope

The simplest spectroscope just requires light from a thin slit to be reflected off a CD.  I think it’s still technically diffraction, but if I say reflection it’s easier to imagine what I mean.  A nice simple design is provided by Cool Cosmos, and involves using a cardboard box with a slit made from tin foil (my version is shown in the image below).  There are quite a few similar designs in this style, including a tube design from Exploratorium and a large box version from Sci-Toys.  You can just pick a design that matches whatever is sitting in your recycling bin.

Spectroscope using carboard box

Shown below are the results that you can expect.

Two sample spectra reflected off a CD. Sunlit clouds (left) and desktop lamp (right)

Alan Schwabacher’s Design

For this design you’ll need to print out a template; cut, fold and glue it together.  I used a file hosted on the American Museum of Natural History; but a higher quality version (and handily three to a page for groups/clubs) is available through this link.  Both designs are attributed to Alan Schwabacher.

The card needs to be black on the inside to reduce the amount of light bouncing around.  I managed to find some large sheets of card in a local craft store that was black on one side and white on the other.  It just needed to be cut into A4 sheets to fit through the printer.  You also need to cut a sliver of CD to fit inside.  I was sceptical that you could just use a pair of scissors, but it turns out you can.

Spectroscope built from folded card and CD sliver

The images below show the results you can expect.  In truth, you get slightly better results since it’s easier to get a good view with your eyes than with the camera.

Two sample spectra diffracted reflected off a CD. Sunlit clouds (left) and desktop lamp (right)

Camera-Mounted Spectroscope

Public Lab has released a design that you attach to your smartphone camera or webcam.  As with the design above you’ll need to print it out on card and assemble it.  Unlike the design above it relies on diffraction through a material, instead of being bounced back.  The ideal design uses a diffraction grating film, which I just picked up from amazon.

Spectroscope built from folded card and diffraction sheet

It is possible to use a CD (or DVD) instead of a diffraction grating film – you just need to remove the label.  The trick is to score a line using sharp blade, and then use tape to peel it off.

Removing the label from a CD using tape

The image below shows the result of using a CD.

Two sample spectra diffracted through a CD. Sunlit clouds (left) and desktop lamp (right)

The image below shows the result of using a diffraction film.

Two sample spectra diffracted through a grating film. Sunlit clouds (left) and desktop lamp (right)

Interpreting the Spectrum

Just looking at different spectra was enough for me; but if you want to go a step further you could consult something like the periodic table of spectra to try and work out the composition.  The Public Lab spectroscope is also designed to work with the SpectralWorkbench – where you can upload your images for analysis.