• Demonstration Highlight: Tyndall's Experiment

    Today we’re featuring an interesting experiment that explores the classic question of springtime: Why Is The Sky Blue?

     Demonstration M7-31: Tyndall’s Experiment uses a chemical reaction to simulate light scattering through Earth’s atmosphere, but in a tiny container. Solutions of sodium thiosulfate and hydrogen chloride are dissolved in water, then mixed together in a glass tank in front of a light source. These chemicals react and form new particles. The resulting particles form a colloid, a liquid with particles suspended throughout, giving it optical properties that let it simulate our much thinner atmosphere on a smaller scale. Over the course of several minutes, as more and more particles form, we see more and more light scattered out the sides of the tank, while less and less passes straight through. As it goes through this transition, the light coming out the end changes from nearly white, to orange, to red, perhaps finally vanishing entirely.

     Demo M7-31: a light source and a glass tank for chemical mixing

     In the atmosphere, light scatters off of molecules and other tiny particles in the air. The angle at which light scatters depends on the wavelength of the light; shorter wavelengths scatter farther than longer ones. Visible light consists of a spectrum of various wavelengths, with blue light having shorter wavelengths and red light having longer wavelengths. This means that blue light scatters more than red.

    Linear visible spectrum, with wavelengths in nanometers - public domain image from David Eccles of Victoria University of Wellington

    This property in the atmosphere was written about by physicists John William Strutt, Lord Rayleigh, and is commonly called Rayleigh Scattering as a result; but the experiment to observe similar effects in a liquid was developed by physicist John Tyndall, hence the name of the demonstration. John Tyndall was well known in his time for his interest in education, and gave public lectures with demonstrations, much like we do today! He also carried out important research on the effect of carbon dioxide in the atmosphere, what we now know as the greenhouse effect.

     Rayleigh scattering in the atmosphere: public domain image credited to Rnbc

    As the Earth turns, the angle we see the sun at changes. Early and late in the day, we’re seeing the sun through the atmosphere at a different angle, and looking through more atmosphere as a result. So at those times we mainly see red light; the shorter blue wavelengths have been scattered away. At midday, with the sun shining directly down on us, we see more blue!

     A fascinating bit of physics, and also very beautiful.


  • STEM News Tip: Hurricane Ida

    Hurricane Ida is a remarkably dynamic storm, and proving hard to predict. Scientific American has a report:

    Meanwhile, keep an eye on for the latest alerts on Ida and other storms to come.

    Other useful links:

    UMD Inclement Weather Guidance:

    Capital Weather Gang:

    UMD Weather

    Jordan Tessler @TerpWeather




  • STEM News Tip: Meteor Showers and Rain Showers

    There's a lot happening overhead this week!

    The Perseid Meteor Shower is at its peak this week. Meteor showers are a stunning sight in the night sky, and the Perseids are one of the brightest of the year. The sparks of light you see in the sky are the burning debris from past passages of Comet Swift-Tuttle, which passes through the solar system every 133 years... but we pass through its path in August every year! Read more about the Perseids at NASA's Solar System Exploration pages.

    Ordinarily, this would not have been the best of years to see the Perseids anyway, since the moon is full, and its bright light would make them hard to see. But it has turned out to be a moot point in our region, as our skies are darkened by a phenomenon much closer to home: Tropical Storm Isaias. This storm has been making its way up the coast and is now dumping quite a lot of rain on us, so everyone please be cautious! These storms can be dramatic to watch, but also potentially dangerous.

    Hurricanes and tropical storms are driven by converging winds and moist air over warm water. Climate change in recent years has warmed the waters of the ocean - by only a small amount, but in a large and complex system, a small change in initial conditions can have a dramatic effect on the outcome! You can see this modeled in the classroom with demonstrations like our chaotic pendulum, G1-60; be sure to check out the simulation linked there that lets you experiment with tiny changes in the initial conditions. 

     Keep your eyes on the sky - but be careful out there!

    Read more:

    National Hurricane Center

    NOAA video: Fuel for the Storm

    NASA: In Depth on the Perseid Meteor Shower Perseid Meteors 2020

    Double Pendulum Simulator by Erik Neumann


  • STEM News Tip: New Atmospheric Phenomena Spotted on Venus

     A recently released paper reports on data from the Akatsuki orbiter. A massive circulating cloud pattern has been identified that has been sustained over the planet’s surface for several decades, but has only just been definitively located. It resembles an enormous weather front but requires further study to fully understand the phenomenon.

     Read more about it:

    Peralta et al: A Long‐Lived Sharp Disruption on the Lower Clouds of Venus

    JAXA: Venus Climate Orbiter Akatsuki

    EarthSky: A deep, giant cloud disruption found on Venus