picHaving recently had conversations about photography, light and colour and been introduced to the idea of spectroscopy as a hobby and a means of understanding the nature of these things, I became curious to know more about how they work and what they are capable of. Although scientific grade spectroscopes can be quite expensive I was surprised to learn in the course of my research that it is quite possible to construct a perfectly adequate and relatively inexpensive one using readily available household objects, namely old shoe boxes, corn flakes packets, empty toilet roll tubes, an old CD, some cardboard and glue. I later learned of a more robust and sophisticated type which could be made from PVC plumbing pipes and at this point became quite excited that even I could make one of these without the worry of it falling to pieces on the first outing. I was confident all the stuff would be readily available at B & Q, but first I would need to put forward a strong business case to convince Ann of the benefits and release the funds from the housekeeping budget. My sense of timing wasn't all that good, we had just had a new central heating boiler and three new radiators fitted, making it necessary to re-decorate two bedrooms. The estimate for the job had been a little more than I wanted to pay so I visited Wickes for a radiator output chart and found we could specify smaller radiators and still achieve the output required. If only I'd thought it through! I could have avoided the redecorating! Naturally Ann had different priorities to me and the case for the spectroscope was falling on deaf ears. The more I tried the more pathetic my argument seemed, reluctantly I had to agree to complete the decorating before submitting a new application with substantial cost savings. I hadn't the heart to admit I had already ordered the diffraction gratings and still clung to the forlorn hope that I might be able to carry on with the project in secret. Just one thought did occur to me at this point, how was I to conceal the delivery of a 3 metre length of plumbing pipe? "Oh what a tangled web we weave when first we practice to deceive", any man who thinks he can pull the wool over his wife's eyes needs his head examining. The project had to be abandoned when the diffraction gratings were delivered unexpectedly early whilst I was at work, they were immediately confiscated not to be returned until the decorating was done. By this stage I was obsessed with building the spectroscope and have to admit the motivational effect was overwhelming. I set to work with a will, just where the energy came from I still don't understand to this day and I'm quite proud to say they're probably two of the best jobs I've ever done. Ann has been telling all the neighbours and letting them into the secret of husband motivation, she's never known me move so fast. However with my diffraction gratings finally restored I set about obtaining the plumbing parts only to find the pipes and bends specified are not the ones normally stocked by hardware outlets. I needed 2" or 50mm pipe and fittings with push fit joints, the fittings being a 22.5 degree bend, a straight connector and an end cap. 32 and 42mm pipe and fittings seemed to be freely available off the shelf but after pondering for some time whether these would do the job I decided to avoid unnecessary risk and stick to the original plan. Benchmark Building Supplies of Romiley were very helpful and obtained the materials for me but I had to take a full three metre length of pipe and settle for a 45 degree bend rather than the one specified. Although not ideal I had been assured this would work satisfactorily in the spectroscope.

  • After buying a selection of black paper and cardboard from Hobbycraft construction was quite simple:-
  • Cut two cardboard rings, outside diameter to fit the inside of the 45 degree bend
  • Sandwich the diffraction grating between the cardboard rings and glue in place
  • When the glue is dry trim off the excess diffraction grating
  • Using a pipe cutter or saw cut a 15" section of 2" pipe and smooth the ends
  • Line the section of pipe with black non-reflective paper
  • With a Stanley knife cut a 1.5" (4cm) slot in the end cap along the diameter
  • Open the slot out to approx 1mm using a nail file and remove the burrs
  • Assemble the mounted diffraction grating against the inside flange of the bend
  • Hold the grating in place by inserting one end of the 2" pipe
  • Attach the slotted end cap into the straight connector and fit to the other end of the 2" pipe
  • Rotate the diffraction grating and the end cap until the slot and the lines in the grating are aligned and a spectrum appears when viewed from the open end.

picWhen set up correctly it should be possible to analyse the light emitted by or passing through various materials, the spectra to the left are early images produced by the finished spectroscope. The graph below is an analysis of the first spectrum representing the wavelength and intensity of the various coloured bands as appropriately spaced spikes (n.b. the wavelength axis is non-linear). Just two things could have been done better - a) although I managed to prevent contamination of the diffraction grating during the gluing process I did cause slight wrinkling. b) the slot cutting didn't exactly go to plan, the finished slot has a slight curve even though I used a steel straight edge as a guide. Neither seems to affect performance over much. By the way, in case you are interested the surplus water pipe will be used to replace the waste water downspout from the bathroom, it's just the right length.

Project Pictures

  • 45 degree bend

    Compatible with 2" plumbing pipe.

  • Connector and end cap

    Compatible with 2" plumbing pipe.

  • 15" section of tube

    The main body of the spectroscope.

  • Mounted diffraction grating

    Glued between two black cardboard rings.

  • Spectroscope assembled

    Diffraction grating mounted between body and 45 degree bend.

  • Energy saving light bulb

    Spectrum photographed using Sony  Cybershot.

  • Tungsten light bulb

    Spectrum photographed using Sony Cybershot.

  • Energy saving light bulb spectrum

    Graph showing wavelengths and intensity of  the various bands (scale is non-linear).

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