SAM MORRELL


ASTROPHYSICIST

Rainbow in a Box

June 23, 2015   |   Reading time: ~5 mins

Yesterday, I helped out at the physics department with a visit for a local sixth form. The basic idea was that in the morning the groups of students got a tour around the building and then in the afternoon they would get involved in a load of experiments. I was assigned the job of looking after the department’s new cloud chamber. For those that don’t know, a cloud chamber is an ingenious device that uses evaporated alcohol to show the trails or ionising radiation as it passes through the box. This includes \alpha and \beta particles, as well as other charged particles. This is a great demo as it actually allows you to visualise the otherwise invisible background radiation that is constantly bombarding us every second of every day. However, the real winner for me was the astrophysics demo. This had the students making a basic spectroscope out of a piece of paper and a CD.

What is Spectroscopy?

Put simply, spectroscopy is the process of capturing and studying spectra. This is a very important scientific method for both chemists and astrophysicists. Due to the way electrons are organised in atoms, and also due to various quantum effects that occur, every atom in the periodic table has a very specific set of frequencies of light it gives off when excited; almost like it’s atomic fingerprint. Not only that, but when you combine atoms together into molecules each one also has its own specific signature. This means that by studying the spectrum of light you can get very useful information about the emitter. Not only that but atoms and molecules also absorb at certain frequencies, meaning that if there are parts of the spectrum missing you can also tell what is absorbing those frequencies. This tool has allowed chemists to discover numerous elements and astrophysicists to study the chemical composition of countless stars and planets that would otherwise be completely inaccessible.

Spectroscopes are the tools used to actually capture these spectra. Back in the days of Isaac Newton, a prism was used to generate a spectrum. This works by taking advantage of the fact that light travelling through a medium, such as glass in this case, travels slower than in a vacuum. It’s at this point we define the rafractive index, which is simply the ratio of the speed of light in the material v to the speed of light in a vacuum c:

(1)   \begin{equation*} n = \frac{v}{c} \end{equation*}

The thing that makes this interesting is that in almost all materials the refractive index depends on the wavelength of the incoming light. In other words the refractive index of material, and hence the speed of light in said material, is a function of frequency. The upshot is that light of different wavelengths moves through the material at different speeds, causing any light non-monochromatic travelling through the material to spread out into a spectrum containing its components parts. This is the essential theory as to why light passing through a prism spreads out to form a spectrum. In modern physics prisms have been largely replaced with diffraction gratings, which work by using diffraction to disperse the light. This is how these really neat little spectroscopes work.

Building a Spectroscope

Luckily there’s no work involved on my part because the Royal Society of Chemistry (RSC) have already done all the work. You can find the PDF with instructions and a template right over here. You’ll need some black card, some tape or glue and a CD. You’re best off following the instructions on the PDF for the rest. When you’ve done you should have a little box with a CD sticking out and a few holes in. There’s a little more to it than that though, there’s actually some fairly complex optics going on which I’ll take you through now.

Point the slit on the front of the box at the light source you want to study; but never directly at the Sun. The best tip is to point it at a white wall which the Sun is reflecting off of, you’ll get the same effect without the downside of blinding yourself. You’ll notice that, when you’ve got it in the right place, you see a spectra form on the CD. What’s happening is that the slit at the front is acting as a collimator, making sure that the light entering the box is in a coherent beam. It also causes diffraction, so if you move your eye around you’ll see 1st and maybe even 2nd order bright fringes, typical of single-slit Fraunhofer diffraction. After the slit we now have a coherent beam of light, the dispersion into a spectrum is done by the CD. The CD’s surface contains tracks that the data on the CD is written along, which double as the grating. The way these are all arranged within the box allow the collimated light to strike the CD at the right angle for the dispersed reflection to travel into your eye. Pretty neat, huh!

That’s more or less it. I may write about some of the interesting features you can see in a spectrum at some point, but this is a really good way to make your own simple spectroscope. I’d like to thank the amazing Natalie Whitehead for masterminding yesterday’s event and letting me be a part of it, and the also amazing Elisabeth Matthews for running the spectroscopy experiment and letting me play with the spectroscopes in the quiet periods.

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