
This window provides a close up simulation of the interactions between
photons and atoms in a gas.  The principal interaction illustrated is
the absorption and re-emission of photons by atoms.  Since this
re-emission sends the photon in an arbitrary direction, the process is
often referred to as scattering.  The following features are provided to
control and vary the simulation and record the results:

Menu Selections

    View - The user can select a large scale view showing many atoms or
    a close-up view of 15 atoms.

    Photon Type - The photons may be either "Line" or "Continuum".  Line
    photons have an energy that corresponds to an available energy level
    in the atoms.

    Quit - Terminates the simulation and closes the window.

Control Buttons

    Step - Sends one photon through the field.  The photon enters the
    field from the left and is initially moving horizontally across the
    field.

    Run - Issues another photon whenever one leaves the field.  When
    Run is selected the Stop button is activated, all other buttons
    and all menu items except Quit are deactivated.

    Stop - Stops issuing photons.  The current photon remains active
    until it leaves the field.  When Stop is selected, the other buttons
    and all menu items are activated.  The Stop button is deactivated
    until Run is pressed.

    Reset - Resets the counters to zero.

Display

    Photon Type - The current photon type (set via menu).

    View - The view currently displayed (set via menu).

Counters

    Photons Sent - A running total of the photons sent.

    Scattered - A running total of the photons experiencing one or more
    interactions with atoms in the field.

    Interactions - The number of interactions experienced by the last
    photon to leave the field.

    Max Interactions - The maximum number of interactions experienced by
    any photon since the counts began.

    Running totals are maintained until the counters are reset.  Note
    that the Scattered, Interactions, and Max Interactions counts are
    not complete until the last photon leaves the field.


