Electron in a "Finite" Square Well

Height of the potential walls (0-2000 units)

Width of well =    


In exercise is designed to study the finite square well and to show the shooting method works. Although the finite square-well potential problem is more realistic then the infinite well, it is difficult to solve because it yields transcendental equations.  With the finite potential, it is possible for the particle to be bound or unbound.   A bound level is one whose energy is less than the well depth.

There are several important features to note:

  1. The wave function is plotted with it's "zero line" rising along the vertical axis (to correspond to the shifting to the energy eigenvalue).

  2. The walls of the graph are hard, i.e., the potential at the walls is always infinite. (So this is a "finite" well, inside of an infinite well!

  3. It will always be possible to get a mathematical solution to the differential equation, but the important question for a physicist is "Does the solution have physical meaning?"  Solutions will have physical meaning if they satisfy the boundary conditions.

  4. Left-click in the graph for graph coordinates.
    Right-click in the graph to take a snapshot of the current graph.
    Left-click-drag the mouse inside the energy level spectrum to change energy levels and wave function of the particle.


Explain how you can observe that an energy value is acceptable as you right click-drag.

Answer: Right-click and drag in the energy spectrum and you should observe the right side of the wave function flipping from negative to positive.  This is a sign that you have passed through a physical solution to Schroedinger's equation, i.e., that the boundary condition at Y( x = +/- 1.00) =0. The correct boundary condition is assumed at the left side of the graph and the wavefunction is calculated in the direction of increasing x.


By changing the principal quantum number, determine the bound state energies for this well.  How do the energies corresponding to the same quantum number compare for finite and infinite potential wells?


Determine the number of  bound states for this well.  A solution of Schroedinger's equation for this problem indicates that the total number of bound states is the next largest integer above the product of the width divided by pi and the square root of the depth.  (Note the scale of the vertical axis in the graph.)  Does this hold true for your results?  Identify in the equation for the potential the parameter that determines the width of the well and make it half as wide.  Does the number of bound states still equal the predicted value?


Decrease the depth of the well to 200 units while keeping the bottom of the well at 0 units.  Are each of the boundary conditions stated in #3 of the Introduction satisfied?   Is it possible for the particle to exist outside of the well even if it's energy is less than the well depth?


Does your conclusion regarding parity for the infinite square well still hold for the finite square well?


What do the wave function and the energy levels look like if the energy of the particle is much greater than the well's depth.  Notice the behavior of the wave at the right boundary.  Describe the effect of the well on unbound wave functions as the energy is decreased.


How do the spacing of the energy levels compare here to the infinite square well energy levels, or to the quantum harmonic oscillator?

Related physlets : quantum harmonic oscillator and the infinite square well

Credits : Physlet problems authored by Dan Boye. Modified by Scott Schneider.