A Potpourri of Physics-y Thought!!

Mars' Sky. Photo from https://mars.nasa.gov/MPF/science/PDS/pds-jun99.html

Dear (Nonexistent) Readers!

This is just a random assortment of physics fun; an email I composed to a student today. Thought y'all might like it too!

• Radioactivity
• Big idea here is that all heavier elements are naturally radioactive. (More specifically, all elements heavier than Iron and even some lighter than Iron). Why? Because the protons in the nucleus repel one another and the larger the nucleus gets, the further protons are from one another. If protons are sufficiently far from one another, their electrical repulsion will dominate over the "strong nuclear force" attraction.
• Neutrons act as nuclear "glue", helping larger atomic nuclei stay together. Why? Because they participate in the strong nuclear force but not the electric repulsion (since they're uncharged). That's why larger atoms have a way higher ratio of neutrons to protons than smaller atoms do.
• Further reading
• Nuclear Fusion
• Fusion is coming together. Happens at the center of stars. Energy is released because some of the original atoms' mass is converted into energy. You can figure out how much energy is converted with the equation E = mc^2.
• Though you don't need to know this for your class, I think it's fascinating to learn about how stars release their energy! If you're curious, read on: In the centers of sun-sized stars, the fusion process is called the proton-proton chain reaction (I didn't say the name yesterday). The reaction in shorthand notation is: 4p → 4
  He
   + 2e⁺ + 2ν_e. What this means in English is that four Hydrogen protons get converted into a Helium nucleus with 2 protons and 2 electrons, 2 positrons (like electrons but positive charge), and 2 electron neutrinos (don't worry about what those are). If you sum the mass of everything on the left side of the equation and compare it to everything on the right side of the equation, you'll find that there's slightly more mass on the left side! The missing mass gets converted into heat and light (which, you may already be aware from personal experience, that the sun produces a heckuva a lot of!), the amount of which you can calculate using E = mc^2!
• Read this for more on Fusion!
• Nuclear Fission: The breaking apart of a nucleus. Nuclear reactors generate energy this way. They basically throw a neutron at unstable heavy elements (like Uranium or Plutonium), which causes them to break apart. The products of this reaction have less mass than the original atoms, so it releases energy via E=mc^2.
• Blackbody Radiation
• This one is trippy! Everything, yes everything, which has a temperature above absolute zero (which is everything) emits light. This is because all light is created by accelerating electric charges. Could be accelerating protons or electrons or any other particle that has an electric charge. When atoms (which have electrically charged protons and electrons in them) vibrate back and forth, they therefore emit light. The frequency of this emitted light is proportional to the frequency of vibration. Hotter objects vibrate faster. So they emit higher frequency light. Therefore our bodies (at ~37 degrees Celsius/98 degrees Fahrenheit) emit mostly in the infrared. Hotter objects (like a blacksmiths iron) glow red. Even hotter and they glow blue! If you look at the stars at night you'll see that there are some red ones and even a few blue ones. The blue ones are much hotter than the red ones! Isn't that awesome that you can just see it with your eyes?! And don't forget that pit vipers (and other animals) can see in the infrared--just like our night-vision goggles. Also, I didn't mention this and it has nothing to do with blackbody radiation, but many insects (like bees and butterflies) can see ultraviolet light!
• Things don't just emit at one frequency. There are a range of frequencies that they emit at because temperature is a measure of average kinetic energy of the atoms--they don't all vibrate at the same rate. Therefore we get a blackbody spectrum. Meaning a range of frequencies.
• Play with this simulation to learn more.
• Photoelectric Effect
• Photo = Light, Electric = Electricity. So the photoelectric effect is about producing electricity from light.
• Einstein won the Nobel Prize for his analysis of it in 1905.
• The underlying physical phenomenon at play in solar panels!
• Photons have to be of high enough energy (same thing as saying high enough frequency) to eject the outermost valence electron from a metal. This electron is then free to move through the metal, meaning it creates an electric current.
• The "work function" is just a fancy term for how much energy is required to kick out the highest energy (aka valence) electron. If a photon has more than this energy, the extra energy will go into the kinetic energy of the ejected electron, increasing the electric current.
• Play around with this (very well done) simulation. You have to get it set correctly to start seeing electrons being ejected. Really try to understand why you see what you see (eg what causes electrons to move faster/slower, be ejected/not be ejected, etc): https://phet.colorado.edu/en/simulation/photoelectric
• Compton Scattering
• Inelastic collision (kinetic energy NOT conserved). Imagine the pool balls--the ball that gets hit doesn't travel as fast as the ball that hit it. Why? Because some of the first balls' kinetic energy got converted into heat--kinetic energy was NOT conserved.
• Compton scattering is the same idea but with a photon hitting a free electron (or other charged particle). Some of the initial photon's energy is converted into moving the electron. So after the collision the photon has less energy, meaning its wavelength will be longer.
• Resonance
• An extremely important concept to understand. It happens all over the place in the natural world.
• Think of two swings, one with long rope, one with short. If you are pushing a kid, you want to push at just the right time (right as they get to you). If you push at the wrong times, you won't add any energy to the kid swinging and they'll come to a stop. For the long swing, you'll push less often because the natural frequency is longer (just imagine how a long swing swings!). For a short swing you have to push more often--but still at the right times.
• Rayleigh Scattering
• The type of scattering responsible for our blue sky. The air in our atmosphere is 80% Nitrogen (N2), 20% Oxygen (O2). Both of these molecules have resonant frequencies with light in the short wavelengths (violet and blue). Therefore violet and blue photons are absorbed and re-emitted (in a new direction--hence the scattering) a lot more than longer wavelengths (like red). Since our eyes are more sensitive to blue than violet (we have three types of cone cells in our retinas--red, green and blue--named after which frequency that type of cell is most sensitive to--we don't have violet cone cells). Also, I assumed in this explanation that you knew violet light is of higher frequency than blue light.
• Mars' sky is red (and the sun turns blue at sunset!). Why? The atmosphere is very different than Earths! There are a lot of tiny dust particles suspended in the Martian air which are about 1000 times larger than N2 and O2 molecules. This means that the size of the particles interacting with light are close to the wavelength of visible light itself--and a different type of scattering (called Mie scattering) occurs. Mie scattering scatters longer wavelengths (eg red) more uniformly than short wavelengths (blue), so Mars' sky looks red (actually salmon)! At the top is a real photo of the sky on Mars!

Let me know if you have any questions and hope you keep being curious about/learning physics even after your class ends!

Sincerely,

Elan