One valid way to think about light is as a stream of particles: photons. Photons are massless, indivisible "chunks" of light energy, traveling from the light source to your eyes at about 3.0 x 108 m/s through vacuum or through air.
Unfortunately, a second valid way to think about light is as an electromagnetic wave, a spread-out pattern of oscillating electric and magnetic fields, analogous to the spread-out pattern of crests and troughs that make up a water wave at the ocean. This is "unfortunate" in the sense that it makes absolutely no sense to say that something that's a particle can also be a wave: a particle is a localized entity, something that is at a particular location at any given moment, whereas a wave is a spread-out entity which therefore is not confined to any one location. Aren't these two descriptions logically contradictory? Yes. Logic aside, experiments show that sometimes you need to think of light as a wave, and other times it's best to think of it as photons. Nature appears not to care whether or not she makes sense to us.
Nevertheless, there's an important relationship between the wave model and the particle model:
Light that can be described as a high-frequency, short-wavelength electromagnetic wave can also be described as a stream of high-energy particles.
So instead of describing violet light as a high-frequency, short-wavelength pattern of oscillating electric and magnetic fields, we can describe it as a stream of high-energy photons. Of course, violet photons are only energetic by comparison to other visible light photons; X-ray and gamma ray photons put them to shame.
Solids, liquids, and very dense gases (stars) tend to be opaque, so they emit a continuous "blackbody" spectrum -- in ordinary English, a rainbow spectrum. (This phenomenon is called "incandescence.") Blackbodies put out more and more light as their temperatures rise, and also put out their peak emission at shorter and shorter wavelengths. Even an opaque object that you typically wouldn't think of as hot -- say, your own body -- emits light, but it does so only at long wavelengths (infrared) that the human eye can't see.
Tenuous gases are transparent rather than opaque. Such gases exhibit emission lines when heated up, emitting light only at certain special wavelengths. Please note that the terms "emission" and "emission lines" aren't synonymous: for example, we saw above that blackbody emission is continuous emission, not emission lines.
We can understand emission lines by supposing that the electrons within atoms and molecules are only "allowed" to have certain energies, and that they can be boosted up to high energy levels through collisions with other atoms in a hot gas. These electrons can now spontaneously drop to lower allowed energy levels, so long as the energy lost by the electron is given to something else. In particular, the something else might be a newly created photon. Since only certain special photon energies are possible -- corresponding to the differences between the electrons' allowed energy levels -- only certain special wavelengths (colors) will be present in the spectrum.
I repeat, the energy of an emitted photon is equal to the downward change in the atom's energy. Furthermore, since different types of atom (helium vs. sodium vs. neon) have different sets of allowed energy levels, different atoms produce different types of photon, yielding different emission-line spectra with bright lines with different sets of colors. You saw this in lab.
It's now a simple matter (I hope) to consider a cool transparent vapor, one whose (sluggish) atoms seldom have energetic collisions. These atoms, then, will typically be in low allowed energy states. What if we now pass photons -- light -- through this gas? For simplicity, imagine that this is white light, a mixture of all possible colors in the rainbow (i.e., a mixture of photons of all possible energies). Most of these photons will pass through the atoms of the gas unhindered, and can be viewed by us on the other side. But certain photons will have just the right energies to boost the atoms' electrons from their low energy levels to higher allowed energy levels. These photons will be absorbed by the atoms, and hence will not be seen by us. Since we see the absence of light as black, we'll see a rainbow spectrum with black lines -- absorption lines -- at certain special wavelengths.
If you've followed the physics, it should be clear to you that the energies of the absorbed photons just discussed are exactly the same as the energies of the emitted photons three paragraphs back. Conservation of energy dictates this: whatever energy an electron loses, the photon must gain, and vice-versa. Thus the set of colors present in the emission-line spectrum of, say, hot sodium vapor is exactly the same as the set of colors missing from the absorption-line spectrum of cool sodium vapor.
Solids and liquids -- condensed matter whose atoms and molecules are close together -- behave similarly, except that their energy level diagrams are extremely complicated. In particular, they tend to have large numbers of closely spaced allowed energy levels, or energy bands. This yields absorption spectra which have large numbers of closely spaced absorption lines: absorption bands. In other words, solids and liquids absorb broad ranges of the spectrum when light is passed through them, unlike tenuous gases which absorb at sharply defined wavelengths. For example, the blue food coloring you saw in lab is a molecule with an absorption band covering the entire red and green portion of the visible spectrum; hence only blue light was able to pass through unimpeded. Similarly, the chlorophyll found in plant cells has a pair of absorption bands, so that red and blue sunlight are absorbed (with the light energy converted to chemical energy via photosynthesis). Green light is not absorbed by chlorophyll, so it instead reflects to your eyes; this is why grass is green.
We can use this information to understand light absorption by photovoltaic (PV) cells (a.k.a. solar-electric panels or, less precisely, solar cells). We're dealing with solid silicon, so we have permitted energy bands separated by a "band gap." An electron that absorbs a photon whose energy is at least as large as the size of the band gap will be able to make a quantum jump to the higher band: the light energy is converted to the energy of a moving electron. Of course, moving electrons constitute an electric current. So you collect these electrons, send them down a wire to pass through your light bulb or washing machine or electric motor, and then deposit them back into the silicon.
There are problems with this idyllic picture. One problem is inefficiency. Photons that aren't energetic enough can't be absorbed; photons that have more than enough energy to boost electrons "across" the band gap are absorbed with energy to spare, but then the electrons tend to squander the excess energy by warming up the silicon. Electrons might fall back into "holes" (gaps in the crystal structure -- see below) before they can be collected, an undesirable effect called "recombination." The metallic grid that collects the electrons has the unpleasant side-effect of blocking some of the sunlight from reaching the silicon.
Additionally, you reduce the available light energy if the panel is not pointing straight at the Sun -- and of course that's not easy because the Sun moves westward every day and moves northward and southward every year. Last but certainly not least, there are these two complications called "clouds" and "night."
So, here's a good review question for you: How do we deal with any or all of these problems -- especially the clouds/night problem?
There's also a physical complication: the electrons in a pure silicon crystal generally won't absorb photons because these electrons are happy staying put within the crystal pattern. So we don't use pure silicon, we instead use doped silicon, a piece of n-doped silicon in direct contact with a piece of p-doped silicon. (You should know what those terms mean.) The "p-n junction" where the two differently doped pieces of silicon come into contact is also called a diode, and in fact a photovoltaic cell is essentially a light-emitting diode (LED) run in reverse. The n-doped side has some extra electrons that can be easily made to move, while the p-doped side has a deficit of mobile electrons, gaps in the crystal pattern which we can think of as positively charged "holes." It is the doping process that changes a semiconductor like silicon from a good electrical insulator to a material that, under the right circumstances (in a PV cell, the right photon energy; in a computer, the right applied voltage), becomes a good electrical conductor. You should also recall how the p-n junction serves to prevent electron-hole recombination from occurring.
Another review question, which partly overlaps the previous one: What else besides the PV panels themselves does your home need if you want to get your electrical power from solar energy?
Pricing plays a significant role in determining how readily people shift to solar energy. Thus you should take a look at the article describing how some utilities are trying to boost prices to restore fairness (as they would argue) or to stifle competition (as solar enthusiasts would argue), and think about whether battery backup is a better option than grid intertie (as Tesla would argue).
Wind is, in principle, a straightforward way to generate electrical energy: use the wind to spin a turbine, and use the spinning turbine to run an electrical generator. How does the available amount of wind power depend on the size of the turbine blades and on the speed of the wind? What restrictions does this place on where one might locate an economically profitable "wind farm"? What limits our ability to use wind power generated in one area to supply electrical power to a different area?
(Incidentally, tidal power is just a variation on wind power: the back-and-forth motion of coastal water is used to spin the turbine.)
You might think that a completely renewable source of energy would be uncontroversial. That's more or less the case in Denmark -- which generates 40% of its electrical energy via wind -- and in Germany, but not in the U.S. Even our leading environmental groups can't agree on whether or not wind power is a good thing. You should be able to use our readings to address the pros and cons, both real and perceived, of wind power.
Since we covered this so recently, I'm not going to write a lot here. Remember to keep three different "nuclear" processes separate in your minds: fusion vs. fission vs. radioactivity. Hydrogen bombs and stars use fusion; atomic bombs and existing nuclear power plants use fission; radioactivity is a spontaneous, naturally occurring process that keeps geology active and allows smoke detectors to work but which is one of the nasty problems with the waste products of nuclear power plants.
On radioactivity, what's a half life? Where does E = mc2 come in? On fission, which isotopes are used in existing power plants? How does a fission chain reaction keep itself going? What factors influence whether the reaction is subcritical vs. critical vs. supercritical, and which do you want for a power plant? Once a chain reaction is set up, how does this enable the nuclear power plant to generate electrical energy? What are the pros and cons of nuclear energy? Which radioactive waste products are the biggest concern? What's a molten salt reactor and why are some people trying to commercialize this design?
Nuclear energy represents a complex situation where energy policy, climate policy, waste disposal, new reactor designs, reactor safety, proliferation/terrorism concerns, and economics come together. You should be able to give an informed overview of this situation, using the information encountered in class (and not just your preexisting notions) to back up your position on molten salt reactors or on nuclear energy in general.