This illustrates standing waves. You can set the amplitude, wavelength, and speed of an initial sinusoidal wave profile in medium 2.
Then you can set the indices of refraction in media 1, 2, and 3. Medium 1 covers the region to the left of the origin. Medium 2 covers the space between
the origin and whatever thickness you specify. Medium 3 covers the remaining space to the right. The top graph shows the superposition of all waves,
and the bottom one, all the separate waves generated by the multiple transmissions and reflections off of medium 1 and medium 3 (I set the reflected and
transmitted wave amplitudes to decay with each reflection so the wave doesn't grow indefinitely). Hit Load after you've specified the information, to see
the initial wave at t = 0, and then Start, to see the wave at all subsequent times. To simulate closed-closed endpoints, set the n1, n3 >> n2.
To simulate closed-open endpoints, set n1 >> n2 >> n3. And to simulate open-open endpoints, set n1, n3 << n2 (n1, n2, n3 are the indices of refraction
of media 1, 2, and 3, respectively). If you set the wavelength in medium 2 just right, then you should see a standing wave develop :).