Interactive animations of electromagnetic waves

András Szilágyi

Institute of Enzymology, Hungarian Academy of Sciences

Electromagnetism

Maxwell’s equations describe electromagnetic waves but they are hard to grasp intuitively.

In our minds, most of us just have vague images of E and B vectors oscillating somewhere in space.

If we ourselves do not really understand electromagnetic waves, how can we teach them effectively?

Teaching circular dichroism?Important method in biological spectroscopy. CD spectra tell us a great deal of information about the structure of proteins.

Textbook figures on circular dichroism do not really help understand what is happening

Animation makes it clear

)/sin( txAEy Vertically (y axis) polarized wave having an amplitude A, a wavelength of and an angular velocity (frequency * 2) of , propagating along the x axis.

GNUPLOT: equation plotting program

Vertical

Horizontal

)/sin( txAEy

Plane-polarized light

)/sin( txAEz

Right circular

Left circular

Circularly polarized light

)90/sin( txAEy

)/sin( txAEz

)90/sin( txAEy

)/sin( txAEz

Interaction of light and matter: Absorption

)/sin( txAeE xy

Material with an extinction coefficient

The light gets weaker (its amplitude drops)

In Out

Interaction of light and matter: Refraction

In Out

Material with an index of refraction n

The light slows down inside the material, therefore its wavelength becomes shorter and its phase gets shifted

)/sin( tnxAEy

Circular dichroism

In Out

Material having different extinction coefficients for right and left circularly polarized lights: R and L

Plane-polarized light becomes elliptically polar

)90/sin()90/sin( txAetxAeE xxy

LR

)/sin()/sin( txAetxAeE xxz

LR

Circular bi-refringence

In Out

Material having different refraction indices for right and left circularly polarized lights: nR and nL

The plane of polarization of plane-polarized light gets rotated

)90/sin()90/sin( LR txnAtxnAEy

)/sin()/sin( R txnAtxnAE Lz

Circular dichroism AND bi-refringence

In Out

Material having different extincion coefficients AND refraction indices for right and left circularly polarized lights: R and L AND nR and nL

Plane polarized light gets elliptically polar, with the great axis of the ellipse being rotated relative to the original plane of polarization

)90/sin()90/sin( LR txnAetxnAeE xxy

LR

)/sin()/sin( LR txnAetxnAeE xxz

LR

Web Tutorialhttp://www.enzim.hu/~szia/cddemo/edemo0.htm

• I. Basic concepts: Electromagnetic waves and types of polarization • Plane-polarized wave: Horizontal • Plane-polarized wave: Vertical• Superposition of plane-polarized waves: Horizontal + Vertical 45º Plane • Superposition of plane-polarized waves: Horizontal + Vertical Right circular• Superposition of plane-polarized waves: Horizontal + Vertical Left circular• Circularly polarized waves: Right and Left• Superposition of circularly polarized waves: Right + Left circular Plane!

• II. Interaction of light and matter • Plane-polarized wave: Absorption • Circularly polarized wave: Absorption • Plane-polarized wave: Refraction • Circularly polarized wave: Refraction • Circular dichroism • Circular birefringence • Circular dichroism AND birefringence

Disadvantages of non-interactive animations (movies)

• Low graphic resolution, image not perfectly clear

• Tutorial is limited to a set of examples; students cannot explore other possible scenarios

• The effect of changing one parameter or another cannot be discovered

• Student just passively receives the information, instead of actively discovering and exploring phenomena

How to make it interactive?

• 3D programming is advanced stuff• OpenGL, Java3D, etc.: very powerful but the

learning curve is a bit too long for lazy people like me

• Solution for lazy people: Visual Python• Visual Python: very high level language, rapid

development– You create objects (e.g. sphere, box, cylinder, curve,

etc.) which immediately appear and any change in their properties is immediately rendered

– OS-independent; you can create Windows binaries

EMANIM

• http://www.enzim.hu/~szia/emanim

• Public domain software

• Source code and Windows binary available

• In English and Hungarian

EMANIM is an application for visualizing electromagnetic waves. Its main features are:

Live, three-dimensional presentationInteractive rotation and zooming with the mouseContinuous animation of wave propagationAny or both of two waves and optionally their superposition can be displayedVectors of the electric field are shown at two planes intersecting the path of the lightWave parameters such as type of polarization, wavelength, amplitude and phase difference can be interactively variedTo visualize the interaction of light and matter, a piece of material can be placed into the path of the lightProperties of this piece of material such as length, extinction coefficients and indices of refraction can be interactively variedUser-configurable colors

EMANIM can help the user understand a wide range of phenomena related to electromagnetic waves, from the simplest such as a single wave in vacuum to the most complex such as circular dichroism. Its features that promote learning are:

19 predefined parameter sets representing important physical phenomenaA brief explanation of each phenomenon is displayed whenever the user selects a phenomenon from the menuPhenomena from the following basic categories:

Types of polarization (linear, circular)Superposition of wavesInterference of wavesInteraction of light and matter (absorption, refraction)Light in anisotropic materials (dichroism and birefringence)Light in optically active materials (circular dichroism and

birefringence) Hints are displayed to suggest parameters to vary in order to gain a better understanding of the phenomenon at handFive predefined views to show the phenomena from the most optimal "camera angles"

Disadvantages of Visual Python

• Only a limited set of available objects

• Limited widget set, another GUI package is needed for more complex GUIs (EMANIM uses Tk)

• No way to suspend the rendering process, therefore some jerkiness is seen in the animations

Feedback

• Web: hundreds of views of the website per day• Dozens of e-mails with positive feedback, many

of them from teachers who use the tutorial for their classes

• EMANIM has been distributed on various CD-ROMs:– A CD distributed to physics teachers in the UK– A CD included with a textbook for the training wireless

network administrators– ComputerBild Italia, a computer magazine