Lecture Videos

• Light is "electromagnetic radiation" or EM radiation. It is a non-mechanical wave that transmits energy from one place to another. It is "self propagating" meaning that it pushes itself forward. This bit of information is necessary before we proceed to discussing the particle aspect of light because the energy of each photon is proportional to its frequency. Recall that frequency and wavelength are inversely proportional to each other at a given (or in this case constant) velocity.
• What is light?

  Energy is the force of EM radiation that self propagates through space
• Energy = can do work
• EM = electromagnetic
• radiation = traveling energy
• self propagate = pushing itself forward
• By wiggling electrons back and forth, an antenna can create a changing e field. This results in an EM wave
• Light is also a particle
• Lots of microscopic things are "quantized" on the microscopic level
• Quantized, made up of tiny individual particles
• Mass of gold bar?


  Number of gold atoms, each with the same mass
• Heat?


  Each individual atom bumping into something else to transfer TE
• Charge?


  Number of electrons or protons each with the same charge
• In the quantum world, everything is pixelated
• Including light. Particles called photons are tiny packers of EM radiation vibrating with certain frequencies. How did we learn this? Photoelectric effect
• Using monochromatic light:

  • intensity (brightens) and current
  • some colors work, others don't
  • Time lag by electrons depends on color with blue light having fastest electrons (shortest lag) out of the colors that work
• Protons and electrons are attracted to each other
• If the kick upwards isn't hard enough, the electron falls back down
• This means that blue photons have more energy than red photons
• Electrons can only exist at certain energy levels
• Energy of a photon?


  E = hf

  f = frequency of light

  h = planck's constant
• The electron in this hydrogen atom can absorb photons in order to increase its energy level. The key to quantization here is that the electron can only exist in those energy levels. We are going to use numbers for examples here... If the electron is sitting at -13.6 eV (n=1) it could absorb a single photon of 10.2 eV to achieve a total energy of -13.6+10.2 which is -3.4 eV (n=2). With that energy the electron jumps immediately from -13.6 eV (n=1) to -3.4 eV (n=2) by absorbing the photon. It can not absorb 10.0 eV because -13.6+10.0 is -3.6 eV which does not correspond to any energy level.
• Electrons can jump from level to level, and can even skip levels! This means that an electron at -13.6 eV (n=1) could gain 12.09 eV to jump directly to -1.51 eV (n=3). Another way to go from -13.6 eV to -1.51 eV would be to gain 10.2 eV, jumping up to -3.4 eV (n=2), then gain another 1.89 eV to jump up to -1.51 eV (n=3). The electron can not gain 1.89 eV first though because that would result in a total energy of -11.71 eV which does not correspond to any energy level.
• If the energy of a photon is greater than the energy hole that the electron is it, then the electron can be knocked from entirely! For example, if a 14 eV photon hits our electron that is sitting at -13.6 eV (n=1) then that electron will be ejected out of the atom entirely, turning that atom in to an ion. Very high energy photons cause ionization by knocking electrons out of their respective atoms. This is why high energy photons like UV radiation from the sun can damage us!
• Numerical examples aside, the electron needs very specific amounts of energy to jump between energy levels, and by absorbing photons of those exact energy levels it can do just that. If the photon energy is greater than the amount of energy needed to get the electron up to 0 eV, then the electron will be knocked free.
• If electrons can absorb photons to move up in energy levels then can they do the opposite as well? Yup. Electrons at higher energy levels typically don't want to stay there- nature tends to prefer the lowest energy position after all
• (Need an example? Well is it easier to climb up the ladder to a slide or to go down the slide?). Electrons will emit exactly the energy needed to drop down to a lower energy level. That energy will be emitted as a photon! Just like with absorption, electrons can jump down multiple levels at a time or one level at a time, with the only rule being that they emit exactly the right energy to jump exactly from one level to another.
• Finally, I want to mention that there are many types of light source that we use here on Earth. All we need to do to create light is excite electrons within atoms, such that the electrons de-excite while emitting photons. Some ways to do this include incandescence, fluorescence, and phosphorescence.
• Incandescent Light. This is the most simple way of creating light. All you have to do is pump thermal energy in to something until the electrons vibrate up to a higher energy level. They then de-excite by emitting a photon. This is why different temperatures cause different colors when heating up metals (think red-hot vs white-hot), it is part of why stars produce light in the sky, one of the ways that flames and fire make light, and it is exactly how incandescent bulbs create light.
• Incandescent Light. I'm fairly certain that incandescent bulbs are illegal in California now due to their extreme inefficiency. They produce EM radiation at all wavelengths because electrons get excited up to random high energy levels (remember n goes up very high, towards infinity). The problem is that we only use a couple wavelengths to see. Incandescent bulbs produce a lot of heat (infrared radiation) that we don't see and is by extension not helpful in a light source.
• Fluorescent Light. Fluorescence is what we described originally! Photons can excite electrons and then electrons can fall back down while emitting photons. This can be very useful if just the right elements are used.
• Fluorescent Light. In a "fluorescent" bulb electrons are shot from one end of the tube to the other end. This is done in a fairly inefficient method: just getting a filament hot enough to boil off electrons. Those electrons rocket across the tube and hit atoms of mercury gas, exciting the electrons in mercury. Those electrons de-excite emitting a bunch of UV radiation. UV is not visible to us, but it can be used to excite electrons in the phosphor coating of the fluorescent tube. The electrons in the coating then de-excite in many smaller steps, emitting light in the visible spectrum.
• Fluorescent Light. Basically, a fluorescent bulb takes a portion of light that is not visible to us, and makes it visible by letting the invisible photons excite electrons in an atom that has many de-excitation steps to produce visible light. It takes a non-useful portion of the spectrum and turns that light in to visible light.
• Phosphorescent Light. Most atoms are more complicated than our beloved hydrogen. Some even have energy levels where electrons can get stuck for extended periods of time before de-exciting and emitting a photon. This is the principle behind phosphorescence. When you have a bunch of atoms like this and you excite their electrons in to these "metastable" states where they are stuck, they randomly de-excite emitting photons.
• Phosphorescent Light. As they emit photons you end up with a material that can take in energy and hold on to it for a bit while slowly letting it go. This is exactly how "glow in the dark" materials work! Light charges up some electrons that get stuck and gradually fall back down. It is also the method used by most glowing organisms (think of the glowing algae!) to create light.
• Electroluminescence. This one is interesting, and I'm going to over simplify it a bit. The short of it is that by combining some semiconductors in certain ways you can set up a junction where electrons want to flow from one side to another, but can't, until you push them with a voltage. If we use a voltage to pull electrons out of one of those semiconductors it can leave spots for future electrons to fall in to, emitting a photon. 
• Electroluminescence. Think of the hydrogen atom. If we use electricity to take away its one electron, then it will be a sad lonely proton, who wants a new electron. If we supply it with an electron it will quickly pull the electron in (opposites attract via electric force) to orbit at the lowest energy level, n=1. Then we use electricity to take away its new electron, and provide it with another new one.
• Electroluminescence. The newest one falls in to the hole where the original was and the process repeats over and over so long as we continue to use a voltage to take away that bottom electron and let new ones fall in from above. This is a very efficient process- leading to a very efficient light source. This is also why LED lights sometimes have weird colors- they only emit a handful of colors of light at a time, since we are getting the same energy drops and same photons over and over.