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Applets for quantum mechanics

Rating: 5 stars
Used in Course: Not used in course
Submitted by: Donald Hornback (Student), Apr 30, 2001
Comment: This review of Quantum Physics Online (English Version) covers only the section
on wave mechanics. I am a junior undergraduate physics major who has completed
a course in modern physics, and I have also been exposed to material generally
covered in the first few chapters of an undergraduate quantum mechanics text.
My response to this site is extremely positive. The animations concerning both
the wave-packet propagation and the steps and barriers clearly illustrate
concepts that I had not yet been able to clearly visualize. When I take a
quantum mechanics course in the fall and will be more actively studying the
mathematics of the subject, I intend on returning to this site for a
better-informed pass at the wave mechanics applets, as well as the additional
material of the site. Comments concerning the specific material reviewed

Wave-particle duality in a Young slit experiment: I found the animation an
excellent representation of the particle-wave slit experiment and was reminded
of Feynman scratching out probability curves on his blackboard. I cranked up
the rate of the ?lumps? being fired and watched as the curves on the histogram
smoothed out to the nearly continuous distributions expected. Simple and very

Propagation of a wave packet: Having scratched my head more than once trying to
visualize a ?wave-packet propagating through space?, this applet and some
re-reading of textbook material helped me to make some headway on this subject,
at least two-dimensionally. I liked that one could specify to watch either the
absolute square of the wave function or the real part of the wave function.
That the parts of the wave-packet with the greater momentum will propagate
faster causing spreading of the wave-packet makes sense when you can watch it
happen. I like the option of varying the width (from .5, 1,2) of the
wave-packet, assuming that the numbers .5,1,2 correspond to relative widths.
This variable ?width? isnice to show that the more localized the packet is
initially, the quicker it spreads out in space due to the correlation of
increased localization and increased uncertainty in momentum. That the parts of
the packet with larger momentums would propagate with larger velocities is
nicely seen, though I would prefer to see the animation progress a bit longer
than it does. Quibbles.

Propagation of non-minimal wave-packets: Just a variation on the previous,
excellent wave-packet applet. The wave-packet localizes and then spreads due,
as I have read, to the different initial configuration of momenta in the packet.
I have also read that outside measurements will collapse a wave-packet, only
for the wave-packet to then spread out or ?grow? with time, collapsing again at
each additional measurement. How exactly is this phenomenon related to the
non-minimal wave-packet portrayed in this applet?

Steps and Barriers: This is the section for which I have the least exposure and
understanding. Again, the animation of quantum tunneling appears to be a
wonderful treatment of the subject, but since I am not versed in the mathematics
of the phenomenon, in my eyes it still smacks of witchcraft, something I hope
that will pass with further exposure. Varying the step size to watch the
relative amount of the wave reflected and transmitted is a very instructive
exercise, and I look to revisit this applet in the near future when I am better

Scanning tunneling microscope: Our excellent electronics instructor here at
Humboldt State has spoke on this topic a few times, and this applet reflects
exactly what I have already learned on the subject (on a relatively qualitative
level). The applet provides a nice, fairly qualitative look at the subject, and
the interactive part is instructive--calling it a game might be a bit of a

Technical Remarks:

I applaud the design and operation of the applets. While they were fairly
sizable, the load time on my 56K modem was only about ten seconds?well worth the
brief wait. There were a few typos noticed, and the brief encroachment of
French on the English site (e.g. barriere). The author has obviously paid great
attention to detail throughout, both in the programming and in the physics. I
offer only my compliments.