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A rainbow in a box & the ATLAS detector?!



Extended list of materials and tips:

  1. A cardboard box
  2. Thick white paper
  3. Heat-sensitive paper
  4. A prism
  5. Sunlight and shade

Delving a little deeper into the physics:

This video is a modern spin on William Herschel's experiment. [1,2] In 1800, the German-British astronomer was investigating whether different colours of light have different warming powers. So he set up a series of thermometers, measuring the temperature of various spots across a rainbow obtained by dispersing white light from the Sun with a prism. He reported an increase in temperature from violet to green to red... but surprisingly, he also discovered that the largest increase by far occurred after the red band of the rainbow, in a region of space where he could not see any colour. He thus discovered a form of "invisible light", which we now today as infrared radiation. Nowadays, we also know that sunlight contains a lot more infrared than it does visible or even ultraviolet light, and that infrared radiation has just the right energy to boost the wiggling motion of the molecules that make up most materials around us – in other words, to increase their temperature. Those two factors explain why infrared radiation from the Sun induces a dramatic increase of the temperature of the material it is shining on, while red light only produces a moderate increase, green light a minor increase, and so on.

In this video, instead of using thermometers, we use heat-sensitive paper, also known as thermochromic paper – paper for which a change in temperature (thermo) is materialized by a change in colour (chromic). Thermochromic materials are either made out of a heat-sensitive pigment or dye applied on paper [3], or out of a liquid crystalline material like the one used for mood rings sandwiched in a plastic film [4]. In order to conduct this experiment properly, you must select a thermochromic paper which has a dark colour for the average outside temperature in your area, and turns bright once that temperature increases by a few degrees. The positioning of the prism in the box as shown in the video will only cast a nice rainbow on the thermochromic paper when the Sun is high in the sky, around noon.

This demo is, in essence, a DIY particle detector. Your eyes are particle detectors – they detect photons of visible light – but they cannot detect photons with an energy higher or lower than a specific range. Our "rainbow in a box" detector extends that range to lower-energy photons, or infrared photons. At CERN, our particle detectors extend the range in the other direction: they are able to detect very high-energy photons, thanks to detector components known as electromagnetic calorimeters. Electromagnetic calorimeters can be designed in various ways, but they all share two key features: they slow down the photons by having them collide into dense materials, and they record the trajectories of the particles resulting from those collisions. ATLAS' electromagnetic calorimeter has a unique design among the LHC detectors: a millefeuille of metal layers bent into an accordion shape slows down the photons, while liquid argon in the gaps between layers gets ionized by the particles created by those collisions. [5] Ionization creates an electric current which can be measured, and in the end, this is how our high-energy photons are detected! And in fact, detecting high-energy photons is of paramount importance in the LHC, as one of the telltale signs of the Higgs boson is its disintegration into two high-energy photons. [6]

Links for further information :

  • [1] David Sang, William Herschel and the discovery of infra-red radiation, IOP Sparks.
  • [2] James Lincoln, Infrared Light Physics Experiments - AAPT Films, YouTube (2014). A video showing a modern recreation of the Herschel experiment and its newer version with a liquid crystal thermochromic film.
  • [3] Examples of thermochromic inks and pigments from an online store.
  • [4] Examples of thermochromic liquid crystalline sheets from an online store.
  • [5] ATLAS website, Overview of the liquid argon calorimeter.
  • [6] Piotr Traczyk, The Higgs Discovery Explained - Ep. 2/3, YouTube (2020).