• Demonstration Highlight: Cosmic Ray Detector

    Demonstration P4-04, the Cosmic Ray Detector, has recently been upgraded. PhD student Liz Friedman shows it in action in this video.


    When energetic particles from space hit the upper atmosphere, they create a cascade of particles in the air around us. Even a single cosmic ray might create a whole shower of particles in our atmosphere. They’re invisible, but with the right apparatus we can detect them. Each of the two dark blocks in this device is a scintillator. When a particle passes through one scintillator, it makes a tiny spark of light, which is picked up by a sensitive photodetector. By checking for correlations between the two paddles, we can spot which sparks are being caused by particles passing straight through from space.

    These particles stream around and through us all the time, harmlessly; but it’s pretty amazing that we can build a tabletop device that can measure them!

    You can read more about Liz’s adventures in physics, hunting neutrinos in Antarctica, in last summer’s Odyssey magazine

  • Light Up the Night: Neon and "Neon" Lights

    Lit up on a billboard so everyone sees them in neon

    -In Neon, Elton John

    For a century, neon lights have been synonymous – even metonymous – with marketing, entertainment, and nightlife. This month marks the 105th anniversary of neon lighting being patented in the US, so it seems like a good time to explore the physics behind the lights.

     part of a gas discharge tube advertising sign, glowing blue and white tubes

    Gas discharge lighting uses the ionization of a gas to produce light. An electric field is created in the tube that strips some of the outer electrons from the atoms of the gas in the tube. These free electrons flow towards the positively charged anode on one end of the tube, while the now positively charged atoms flow towards the negatively charged cathode. They collide with other neutrally-charged atoms, exchanging charge and gaining energy from the collisions; when these now higher-energy atoms release this excess energy, they produce light. The color of this light is determined by the structure of the atoms; each produces a distinctive spectrum.

     The glow of an actual neon discharge tube, bright red, in a vertically mounted glass tube

    When first created as an experiment, these tubes were filled with a variety of gases, and often burned out quickly. French scientist Georges Claude discovered that using noble gases like neon and argon, which do not react with most other materials, greatly extended their lifespan; and also developed better ways of keeping the electrical power steady in the tube. This made them practical for lighting and signage, and he was able to patent the device;  a patent was approved in the US on January 19, 1914.

     Illustration from Georges Claude neon tube patent, showing a glass tube and power supply; Patent granted 19 January 1914

    Lights like this have been used for many other purposes in the past, as well – many outdoor lamps use gas discharge lighting, though this is now being replaced in many places by more energy-efficient LED lighting. So soon we may only see gas discharge lighting in the laboratory and in artistic applications like these.

     a twisted glass tube glowing in a variety of colors, possibly doped with traces of different elements

    We often use straight tubes with a single gas in them in the classroom to demonstrate the spectrum of an element or compound. Many classes have used these when introducing the concept of spectroscopy or when discussing the structure of atoms; check out demonstration N2-05: Diffraction Spectra, which can be used with lightweight diffraction gratings to let every student see the spectrum of some common or popular elements. This has consistently been one of our more popular demonstrations. Less often used, though, are these classic discharge tubes (sometimes called Geissler tubes) found at P3-24. They can be a fun addition to any discussion of ionization and atomic structure.

     various discharge tubes of a variety of colors and shapes

    Despite the common name, many of these signs today aren’t actually filled with neon, but argon; other gases can be used as well. In the pictures here, only one light has the distinctive red glow of neon. Colloquially, though, they tend to all be referred to as “neon lights,” if only because “gas discharge tube light” is hard to fit into a rock lyric.


  • Phun with Electrons: Particle or Wave?

    Welcome back to the Physics Demonstrations Blog! We’re back from the winter deep freeze and here to share more fun ideas about physics. This week, a brief look into the quantum world with electron beams!

    The electron is a fundamental particle, one of the earliest discovered in modern physics, which has a small but finite rest mass and carries a negative electrical charge.
    However, quantum mechanics has taught us that, under certain circumstances, the electron, which we conventionally think of as a particle, can also behave like a wave.

    a cathode ray tube with power supply

    A stream of electrons traveling through space means a stream of charges is traveling through space – this is, in effect, what an electrical current is. And an electrical current can be deflected by a magnet – so, too, can a beam of electrons. If you hold a magnet near a beam of electrons, like the one in this cathode ray tube, the beam will deflect in a direction perpendicular to the magnetic field.

    glowing cathode ray tube, an electron beam is green against a white screenglowing cathode ray tube, green electron beam deflected by magnet

    Likewise, if you run two electrical currents parallel to each other, the electric field of each will deflect the other slightly, which can be seen in either two wires running parallel, or by running a wire parallel to the electron beam.two parallel wires being pushed apart by the force of the currents within themglowing cathode ray tube, green electron beam deflected by the current in a parallel wire

    This beam is made up of electrons; the glow in the screen behind it is made when a few individual electrons passing by interact with the phosphors in the screen.

    a cathode ray tube with power supply, facing end-on towards the viewer

    But what happens if we take that beam and pass it through a narrow opening? Conventionally, if you throw a bunch of particles at a hole, either they pass through in a straight line, or they miss and bounce off. But instead, look what happens when we do it with electrons:

    cathode ray tube with green electron beam diffracted into ringscloseup of diffraction rings of an electron beam

    The electrons are not passing through in a straight line, but instead are diffracting, forming a pattern of rings. This is an interference pattern formed by the interaction of the peaks of a wave. We can see this frequently when light passes through a narrow opening, forming such a pattern.

    red laser light forming diffraction rings after passing through a pinhole

    But here, it is being formed by electrons – very small particles, but particles nonetheless. Thus, here we see an electron behaving as a wave.

  • Quantum Demonstration and Simulation: The Hydrogen Atom

    We love our demonstrations, but there are some things you can’t easily demonstrate in the classroom, either because the physics isn’t compatible with that environment, or because the scale is beyond what we can practically see. This is where simulations can be valuable, in letting us go beyond what we can do on the tabletop and look inside the black boxes. a glass tube of ionized hydrogen glows faintly in the darkness

    The quantum nature of the hydrogen atom is a good example. We can demonstrate the emission spectrum of hydrogen with the Balmer Series demonstration P3-51, and we have simple models of electron orbitals for more complex atoms, but how can we look at the structure of the hydrogen atom itself?

    Here are some simulations available for looking inside our smallest atom.

  • STEM News Tip: Atom interferometry in free fall

    This spring brings another development in AMO physics. A German research team reports successfully carrying out atom interferometry in a Bose-Einstein Condensate in a microgravity environment using sounding rockets.

     Interferometry uses the interaction of waveforms to make ultrafine measurements. We do this routinely in demonstrations using light waves, but atom interferometry uses the wavelike characteristics of atoms themselves.


    Read more:


    Lachmann et al (2021), Ultracold atom interferometry in space, Nature Communications 12


    Padavic-Callaghan (2021), Ultracold Quantum Collisions Have Been Achieved in Space for the First Time, Scientific American


    Dataset for the manuscript "Ultracold atom interferometry in space" at Leibniz Universität Hannover

  • STEM News Tip: Prof. Alicia Kollár Receives NSF Award

    Alicia Kollár, Assistant Professor of Physics and JQI Fellow, has received the National Science Foundation’s Faculty Early Career Development Award. This NSF program recognizes scientists making important advances in both research and education.

    Kollár’s research will be studying photon behaviour in superconductor circuits, a potential part of quantum computing systems. She will also be developing online outreach programs to introduce more people to the wonders of quantum physics.


    Read More:

  • STEM News Tip: Quantum Fireside Chat with Nobel Laureate Bill Phillips

    On September 22, Prof. Bill Phillips and Dr. Laurie Loccascio will present a virtual fireside chat, “Quantum Basics for the Curious.” This event, hosted by the Mid-Atlantic Quantum Alliance, will explore Prof. Phillips’ research and how he came to win the Nobel Prize, an introduction to quantum mechanics, and the future of quantum physics and technology.

    The Mid-Atlantic Quantum Alliance launched in January with a ceremony in Annapolis; and focuses on cross-disciplinary collaborations in quantum technology. The MQA consortium includes UMD and other USM institutions, and other academic, government, and industrial research centers.

    Read more:

  • STEM News Tip: UMD Tech Spin-Offs Highlighted

    A new article in the National Academy of Inventors journal Technology and Innovation highlights commercial spin-offs of UMD research. The article explores both new exploratory startups, and some already fully active and profitable, and talks about the strategies behind their success. Among the firms profiled is IonQ, the quantum computing company cofounded by UMD Physics’ own Prof. Chris Monroe

    The authors, Julie Lenzer and Piotr Kulczakowicz, are respectively UMD’s Chief Innovation Officer and UM Ventures’ Senior Innovation Manager.


    Read more