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A cardboard spectroscope and antimatter research?!



Extended list of materials and tips:

  1. A cardboard tube (relatively stiff tubes are more convenient)
  2. A linear diffraction grating (those are easily bought for cheap on the Internet or in astronomy stores; we used a grating with 500 lines per millimeter)
  3. Cardboard
  4. Matte tape (tracing paper would work too)
  5. Black tape
  6. Various light sources

Delving a little deeper into the physics:

In this video, we make a cardboard spectroscope, an instrument used to separate light into its various colours. The central piece of the spectroscope is a diffraction grating. This is an opaque screen consisting of multiple elongated holes, or slits, with a fixed spacing in between them on the order of the wavelength of the light being studied. In this situation, light behaves like a wave, and its propagation is perturbed (diffracted) by the many slits, a bit like water waves going around wooden poles on the beach. Different colours are perturbed in different ways, leading to the separation of colours past the diffraction grating. Importantly, colour separation happens at an angle to the light source, not when looking straight at it.

In this design, we use a transmission grating so we can point the instrument in the direction of the source of light. This makes the operation of the spectroscope a bit easier than if the grating were of the reflective type, like a DVD. (In any case, do not use the spectroscope to look directly at the Sun!) We use a "crooked" carboard tube so as to look directly into the direction in which colour separation happens. If we were to use a straight cardboard tube, we would be looking at the light source, and colour separation would not happen at the center of our field of view. Additionally, make sure to use a single-axis (linear) grating, as we want to spread the colours across one single direction. Double axis gratings spread the colours horizontally and vertically, which is not suitable for viewing spectral lines. Lastly, we picked a grating with a 500 lines per millimeter. A higher number would disperse the colours more, so one could potentially distinguish two very close lines with greater accuracy, but colour separation would happen at an angle even greater to the light source, making the instrument a bit more unwieldy.

On the other side of the tube is a slit with matte tape. The tape acts as a diffuser: it makes the incoming light homogeneous and uniform through the slit. The thinner the slit, the thinner the spectral lines, so the more precise the instrument; but also the less light enters the tube, so a compromise has to be reached by trial and error. Some DIY spectrographs designs use two razor blades instead of tape to make the slit, as this creates a very straight and sharp gap – but be very careful if you decide to go that route! You can point the slit at all sorts of light sources. The diffuse light from the Sun on a cloud, the heating resistance of a toaster, or an old-fashioned incandescent light bulb produce continuous spectra. On the other hands, most of the light sources around us these days are LEDs, which show a discrete (discontinuous) spectrum with emission lines, just like fluorescent lights (compact or straight). Higher-quality lights will have "fuller" spectra (more lines, some continous regions) than low-quality lights. Lastly, light sources made of vapours of excited atoms also show line spectra – this can be observed while looking with the spectroscope at glowing neon tubes, or sodium vapour lampposts, or mercury vapour lamps in physics classrooms, or at the xenon lights of a car. And each atom has a unique spectral signature, a bit like an atomic fingerprint.

Physicists have shown that matter and antimatter were formed in equal quantities during the Big Bang, but today, the Universe is dominated by matter. So, where did all the antimatter go? Antimatter particles share the same mass as their matter counterparts, but characteristics such as their electric charge are opposite. Physicists are looking for any more differences between those two types of matter, not described by the Standard Model of Particle Physics, which could explain why we observe an imbalance between the two today. [1] And one such avenue of research is to look at the spectra of antihydrogen atoms – atoms of antimatter made by combining a positron and an antiproton – and to compare them with the spectra of hydrogen atoms – which we know to an astonishing degree of precision. Our current theories dictate that the two spectra should be the same, but finding any differences would point in the direction of clues which could explain the origin of the matter/antimatter asymmetry problem. This is one of the goals of the ALPHA experiment at the Antimatter Factory hall of CERN, where the second part of this video was filmed. [2]

Links for further information :

  • [1] Overview of antimatter research at CERN from home.cern.
  • [2] Michael Charlton et al., Antihydrogen in a bottle, Physics Education 48 212 (2013). An article written by researchers from the ALPHA collaboration for physics teachers laying out the goals, challenges and design of their experiment.