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## Definitions of Important Terms for This Unit (10 minutes)

Presenting these definitions in between students Emulating a Wave in a Periodic System is highly recommended. Doing so will help solidify the connection between the verbal and physical representations.

- Envelope functions:
- Describe the displacement of discrete atoms in an oscillating system. That is,if an envelope function is described by $\psi (x,t)$, the value of the function will describe the displacement of a particle at location $x$ and time $t$ .
**To relate this to the wave emulation, have the students perform the activity in front of a blackboard. Before the students start oscillating, draw the envelope function describing them on the board behind them. Tell them this is the envelope function describing the particles at time $t=0$.**

(As a side note, you can also note that an envelope function of wavelength $a$, where $a$ is the separation distance between each molecule, is equivalent to an envelope function of wavelength $\lambda → \infty $. Have the students perform this scenario if you wish.)

- Normal modes:
- Are a special set of envelope functions.
- Are special
*because*each atom in the system oscillates with the same frequency when that normal mode is excited. - Occur from satisfying particular boundary conditions (fixed, periodic, conditional, etc.).
**To relate this to the wave emulation, discuss the choices of $k$ made for each oscillating system. For example, the instructor can note in some case that the ends of the wave are moving exactly in phase with each other; this satisfies periodic boundary conditions.**

- Dispersion Relations:
- Relate the
*shape of the envelope function*to the normal mode frequency.