Of all the molecular ions that exist, by far the least complicated is the dihydrogen cation (H2+) which is just the hydrogen molecule with an electron removed. The H2 only has two electrons in it normally leaving only one when it is turned into an ion. This means that solving the Schrödinger equation can be solved with quite a bit more ease than in most cases with only the quantum mechanics of one electron to consider. Another feature is that the covalent bond that holds the hydrogens together is only composed of one electron.
An optical resonator can be described as just a box made of mirrors. If strong enough laser light is shone on the outside it will transmit through one of the mirrors and the photons will start bouncing back and forth inside the box, known as a cavity. As all lasers must have some divergence eventually this bouncing light is destined to spread out and fade away. It is however possible to form lasting stable reflections within the cavity and when this happens the set up is known as an optical resonator. This can be imagined as a standing wave of light in a fixed shape. If a gain medium is present then this design can be turned into a laser with the present photons generating more through stimulated emission. When an active medium is present the resonator is called active and unsurprisingly passive is the name given to when there is no contributing medium.
The free electron model is a simple model for electrons in a metal crystal where it is assumed that the electrons become completely separate from their metal ions. This means they can be seen as behaving like an ideal gas with all the qualities that come with it. This means the electrons electrostatic repulsion is assumed to be negligible (hopefully reduced by the positive ions), the crystal lattice is generally ignored except for the boundary. The mass of the electrons is replaced with an effective mass to assist in this analysis with even negative effective mass being possible if it is a hole moving instead of an electron.
Quantum entanglement is when particles are produced which cannot be described as independent units but instead have a quantum state that must describe all of them simultaneously (for anyone good at maths, here’s a more mathematical description). If two electrons are produced that we know must have total spin zero, then of course one will have spin up and the other spin down. This leads to a seemingly impossible occurrence where the particles seem to be able to communicate information about what has happened to them over arbitrarily long distances instantaneously. The experiments have only ever shown this to be reliably true on the microscopic quantum scale, not surprising considering thermal fluctuations reek havoc upon quantum effects when they aren’t properly controlled, which is very difficult as size increases.
Interferometry is a method of investigation where waves, normally electromagnetic radiation, are superimposed to gather information about the path the waves took. If a coherent wave is spit into two perpendicular directions then reflected and brought back together they will constructively interfere if the paths they took were the same and destructively interfere if the paths they took were half a wavelength out. It was this process that allowed us to both disprove the aether argument of electrodynamics and show that gravitational waves existed (as in both cases the existence of these effects would lead to a distortion of a laser passing through space).
Dry friction is probably the friction the reader is most accustomed to. The friction that occurs when two solid surfaces that are in contact are sliding past each other. The ability to control friction is very important. Machine parts, cogs and wheels all need to have as little friction as possible while car brakes and pulleys both require it. Frictional force is normally calculated as simply being the normal contact force (the force the ground exerts on the object) multiplied by a constant. This means that surface area has no effect on frictions as, when you think about it, the rough surfaces of materials only touch in protruding places. But what happens when surfaces that are flat down to the atomic level are allowed to slide over each other?
In chemistry Hund’s rule is one that may not always be remembered associated with his name, but is nonetheless always recalled. It is the fact that electrons will always attempt to avoid pairing with another electron in an orbital and so all orbitals in a shell will gain one electron before they start doubling up. This can be simply explained as this arrangement allows the electrons to exist nearer the nucleus and so makes the overall atom more stable.