This applet qualitatively demonstrates the non-relativistic Doppler effect due waves emitted from a moving point source, including the production of shock waves. The usual parameters (wavelength, wave speed, source speed) can be varied within limits, and the user can measure the distance between wave crests to verify the Doppler formula. The simulation shows how the Doppler effect derives from the various motions involved.
Type of Material:
Lecture/Demo; Homework. The latter will require specific instructor designed activities or assignments.
Browser with Java support.
Identify Major Learning Goals:
To achieve qualitative and quantitative understanding of the Doppler effect. Specifically a user will investigate relations between wave speed, source speed, initial wavelength and how this affect the distance between wave crests.
Target Student Population:
Introductory physics in high school or college.
Prerequisite Knowledge or Skills:
Algebra and study of waves (meaning of wavelength, frequency, wavespeed)
Evaluation and Observation
The qualitative explanation for the change in wavelength is easily seen from the simulation. The item gives a clear visual explanation of why the Doppler effect occurs. The user is able to change several parameters to investigate a wide range of conditions. Wave speed, wavelength, and source speed can be user selected. The simulation can be stopped and restarted with a mouse click, and time is displayed.
While the simulation does an excellent job at presenting an overview, there is no "observer" available in this simulation. The measurement tools allow observations to be made from any point while the simulation is stopped, but the physical presentation of an observer making measurements seems to finish off the topic in the way it is often presented in most texts. This would at least illustrate the two special cases considered in most classes and texts, moving toward or away.
Potential Effectiveness as a Teaching Tool
For a moving point source the simulation displays a topic that is usually difficult to draw on a board in class. In addition to qualitative concepts, a user will be able to make measurements and either derive or verify results of the Doppler effect. The applet can also be used to clearly illustrate the transition to shock waves.
An instructor can use the applet as a lecture/demo to illustrate the qualitative aspects of the Doppler effect. To use the item quantitatively as homework, specific assignments will need to be created. In addition, for self study, the students will need to be told that the circular lines represent (e.g.) spherical wave crests since there is no discussion of the translation from the simulation to an actual moving wave source.
The lack of units in the applet is troubling because it caters to the well-known tendency for introductory students to ignore units.
Ease of Use for Both Students and Faculty
Parameters can be easily changed, and the ability to toggle the motion on and off makes measurement of the wavelength very simple. There are brief but clear instructions that descibe each control and action. Most of the use is self evident in the simulation.