Motion in the Ocean: Waves and Optics in a 3D Virtual Environment

• By Taha Mzoughi, John T. Foley
• 05/30/03

Figure 1: The WEBTOP Waves module simulating the interference of the waves in a ripple tank caused by two small oscillating plungers at the back end of the tank.

The last 20 years has been a transformative period in the field of optics. In that time, an amazing array of technologies have spawned and entered the life of each of us, from fiber optics to optical scanners, digital cameras, CDs, and laser eye surgery, to name a few. Yet new research holds the promise for producing even more break-through tools and technologies, including the trapping and cooling of atoms with laser beams; the resolution of individual atoms on surfaces; optical sensors for diabetes; and perhaps even a quantum computer whose performance will greatly exceed that of current machines.

It makes sense then that science and engineering students should learn optics thoroughly so as to understand important current technologies, to create exciting new ones, and to do the basic research needed to go further on the optics journey. The goal of The Optics Project on the Web (WebTOP) is to help them do this.

WebTOP is a three-dimensional, interactive computer graphics system developed at Mississippi State University to help students learn about waves and optics. It has been used to help teach undergraduate introductory physics and junior/senior level optics courses and is currently made up of 16 modules spanning eight different subject areas: waves, geometrical optics, reflection and refraction, polarization, interference, diffraction, lasers, and scattering.

The simulations in WebTOP are 3D, allowing the user to view the simulation from any point of view. The user can move the scene, zoom into a particular location, and adjust the viewing angle. They are also interactive, with the parameters of the simulation elements easily accessible from within the scene. The user can position the mouse cursor over the element and manipulate the provided widgets to change the corresponding parameter value. The user can also change the parameters by typing desired values into the text entry boxes. Depending on the simulation, the user can also collect either qualitative or quantitative data to verify his or her assumptions. In addition, the simulations are animated, allowing the user to view the time progression of the phenomenon.

VCR-type controls allow the user to record his or her interaction with the simulation for later retrieval and viewing. Each interaction is recorded as a line in an XML file (a script) with a corresponding time stamp. The script can then be later loaded into the module and played. Each interaction is then reproduced at the appropriate time. Like in a VCR, the user can pause, stop, and replay a script. The scripts are human-readable and can be easily modified with a text editor. This is particularly useful when a user wants to set up an intricate situation involving many simulation elements, or when a user wants to "clean up" a previously recorded script. A script can also be used as a parameter for setting up a Web page that invokes a module. When used as such, the script defines the initial state of the module.

WebTOP can be run from our Web site (http://webtop.msstate.edu) or be downloaded and run on the user’s local machine. Each module includes a directions section, a theory section, a list of pre-recorded examples, and a set of suggested exercises for the user. The simulations are implemented using the Virtual Reality Modeling Language (VRML), Java, and the External Authoring Interface (EAI). They run in Microsoft’s Internet Explorer Web browser with the blaxxun Contact 5.0 VRML browser plug-in installed.

Virtual Ripple Tank

Waves, one of the WebTOP modules, simulates a ripple tank. The user can place one or more point sources (a small plunger oscillating up and down) and/or line sources on the water surface. The module shows the resultant disturbance, either as a still picture or as traveling waves. The user can even advance the animation one frame at a time. For each point source the user can interactively vary the amplitude, wavelength, and initial phase of the wave it generates, as well as the position of the source. For each line source the user can vary the parameters.

Figure 2: The Waves module simulating the circular waves in a ripple tank created by a small oscillating plunger. The widgets associated with the source are shown.

The user can change the amplitude of the plunger's motion by pulling on the gold cone on the tip of the pole with the mouse cursor, change the wavelength of the wave by pulling on the blue ring, and change the initial phase of the wave by pulling on the gold collar.

The user can also adjust the viewing angle or the position of the ripple tank, and can zoom into a particular location on the water surface and view the water level rise and fall, as in a real ripple tank. Sampling sticks can be added to the water surface to read the water level at particular points. Like buoys, the sampling sticks rise and drop with the water surface. This is particularly useful when illustrating constructive and destructive interference.

In addition to the typical experiments that are usually performed with ripple tanks, the Waves module allows for more intricate and interesting experiments. For instance, the user can add a virtually unlimited number of sources. This allows the one to illustrate how a plane wave can be formed by a large number of point sources. The user can also set the parameters of individual sources independently of each other, and can combine line sources and point sources.

WebTOP for Teaching and Learning
Originally, the main goal for WebTOP was to provide high-quality visualizations for use in classroom demonstrations. But wave and optical phenomena are noted for being visually complex, and difficult to demonstrate effectively in the classroom. Most available demonstration equipment requires in-class set-up procedures that can sometimes be lengthy and elaborate. Moreover, it is often quite difficult to effectively show the phenomenon and its intricacies to all of the students simultaneously, especially in large classrooms. And most importantly, the students cannot come back later and experiment with the demonstration.

To use the computer graphics system for classroom presentations, the user needs only a computer and a projection system. Preparation can be done before class by recording the actions to be performed as a script that can be played during class. WebTOP’s 3D nature allows the presenter to show the phenomenon from a variety of viewpoints. The ability to change the values of all the relevant parameters in a simulation allows the instructor to show, in real time, the effect of changing each parameter in a way that can be clearly seen by the students, even in large lecture halls.

Using WebTOP provides an ideal mechanism for creating an active learning environment that engages the students. The teacher can ask "What will happen if" types of questions, and get the students to predict the outcomes. The instructor can then show them the outcome and encourage them to discuss why it occurred. Finally, students can access the system on their own, online after class, and experiment and answer their own questions.

Soon after developing some of the early modules, we found that WebTOP could also be used effectively for homework assignments. Students were asked to complete homework sets where they analyzed certain situations, produced numerical or qualitative answers to questions, and then used WebTOP to simulate the situation and check their answers. The students were also asked to turn in their written answers to the questions and the relevant screen captures from the modules or scripts.

The system can be also be used to supplement laboratory activities. It can be used in pre-lab activities to help explain the phenomenon that is going to be investigated. It can also be used to help students prepare to look for and identify a specific phenomenon during the lab (e.g., a faint spectral line that could be easily overlooked). During lab, WebTOP can be used to compare actual data to the simulation results and to help draw inferences when equipment limitations occur.

Polarization Module

This module uses a different representation of monochromatic waves. In this case, the electric field vectors of the wave are shown. The user can choose the incident electric field to be either completely polarized (the orientation in time follows a particular pattern) or to be unpolarized (the oriented in time is random). Two types of optical elements can be added: linear polarizers and wave plates. Linear polarizers are used in sun-glasses to eliminate glare. The user controls the properties of the incident field, and the type, location, and characteristics of the optical elements being used.

In typical demonstrations, the users can only show how polarizers block some of the light. With WebTOP the user can see the effect of the optical element on the electric field vectors, can get a reading of the effect of the optical element on the intensity of the wave, and can combine several optical elements. Furthermore, the user can see the electric field vectors and understand how they evolve in time.

Figure 3: The Polarization module. Circularly polarized light is incident upon a linear polarizer.

In addition, WebTOP can be used to develop online tutorials. Developers can embed simulations into a Web page. With scripts, they can illustrate numerous phenomena by simply setting up different initial parameters or by recording a set of actions. Users can also use scripts and module sizing options to display simple snapshots from WebTOP modules, and to disable some of the interactions.

Another use for the computer graphics system is student projects. Students can work in teams or individually on particular problems and use WebTOP to help investigate and understand the phenomenon. It can then be used by students to help illustrate their presentations and reports.

WebTOP has been proven to be an effective tool for classroom demonstrations, homework, and for developing online tutorials—as well as to help supplement laboratory activities and student projects. However, WebTOP is not a finished product. There are plans to develop more curriculum material based on WebTOP modules and to develop additional modules, as well as to make it an open source program.

For more details about WebTOP visit: http://webtop.msstate.edu.

Reflection/Refraction Vectorial Module

This module simulates a monochromatic plane wave incident upon a planar interface that separates two media, e.g., air and a sheet of glass. The representation of the wave is the same as the one used for the Polarization module. The user can choose the incident electric field component of the waves to be either completely polarized or to be unpolarized. In the completely polarized case, the user can interactively vary the wavelength, angle of incidence of the wave, the amplitudes of the two electric field components and the phase difference between them. The user can also vary the indices of refraction of the media used. The corresponding time-varying incident, reflected, and transmitted (refracted) electric field vectors along the corresponding ray paths are displayed on the screen. The user can select which components of the electric field are displayed. This module is particularly useful for demonstrating polarization by reflection.

Figure 4: The Reflection and Refraction/Vectorial module. Circularly polarized light is incident from air onto water at Brewster’s angle. In this case, the reflected beam is polarized.