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Doppler Effect Demo

Brief Summary

When a sound source emits sound, or a light source emits light, it emits energy in the form of waves. The FREQUENCY of the waves refers to how many oscillations are emitted per second. For sound, the higher the frequency, the higher the pitch of the sound. For visible light, the higher the frequency, the closer the color is to the "blue" end of the spectrum. In 1842, Christian Doppler realized that the frequency that you hear is not necessarily the same as the frequency that was emitted. If the sound source is coming towards you, the frequency (pitch) that you hear is higher than what was emitted. If the sound source is moving away from you, the frequency sounds lower. Although Doppler didn't realize it, an analogous effect is true for light.

This demonstration allows visitors to hear the Doppler Effect, to see an animation about what causes it, and to find out about how the Doppler Effect is used in astronomy to understand the motions of some celestial objects. The presenter of this demo has the option of ending the demo with either ENDING A, explaining the search for planets beyond our solar system, or ENDING B, explaining the idea of the expanding universe.

Equipment Required

Buzzer Ball

Doppler Effect Demo media

Solar Spectrum Chart

Expanding Universe Chart

Star - Planet Mobile (shown)

Expanding Universe Charts

Microphone and PA

Laser Pointer

Main Teaching Points

Sound and light are forms of energy transmitted as WAVES. The FREQUENCY of the waves determines the sound's pitch or the light's color. The motion of the sound or light source can alter the perceived frequency of the sound or light. If

the sound source is coming towards you, the frequency (pitch) that you hear is higher than what was emitted. If the sound source is moving away from you, the frequency sounds lower. An analogous effect is true for light.

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The faster the source is moving, the more the frequency changes from the still observer’s perspective.

By comparing the frequencies of light observed from a celestial object with the frequencies they know the object must have emitted, astronomers can calculate how fast the object is moving toward or away from the Earth.

There are two different endings for this demo, with different teaching points: This technique is currently used to discover planets orbiting around distant stars. (ENDING A) Using this technique, Edwin Hubble was astounded to discover that the universe is expanding.

(ENDING B)

Educational Strategy

This demo provides a high level of sensory saturation that enhances the formation of long-term memories. Visitors can use several of their senses as well as different thinking styles in observing this presentation. They hear the Doppler Effect. They compare it with a visual animation of why the Doppler Effect happens. Even the animated explanation can be understood either analytically (left brain) or can be simply taken in visually (right brain). In the section where you explain how the Doppler Effect is used, visitors can compare the theory with practical applications.

Since this demonstration is done in the round, the audience has more of a street-theatre type of experience than with more traditional set-ups. This adds to the variety of offerings in Space Odyssey.

Set Up

Turn on screens and load Doppler Demo Media Check Buzzer Ball Get out Solar Spectrum Chart and Expanding Universe Chart Get out Star – Planet Mobile

Suggested ways of presenting demo

1. Spin buzzer ball to attract crowd 2. Buzzer Ball

Play buzzer without spinning so that everyone can hear what it sounds like.

Have crowd stand back, then spin the buzzer ball. Be sure you loop the cord over your wrist.

Ask visitor to describe (or, better yet, sing) what they just heard. (wavering up and down pitch)

Ask crowd to take a few steps closer and you will explain how it works.

3. Animation

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Start the “What is the Doppler Effect?” animation. Each of the following steps refers to one of the animation sequences. In other words, after each numbered item, advance to the next sequence. This is a view of a wave as seen from “above”. You can think of it as a water wave, sound

wave, or light wave. (I will use light wave in this explanation.) The tops of the waves are called the CRESTS. We’ll be following the crests in this animation.

Here is a cut-away view of the same thing. The expanding circle represents a single crest moving out from its source. Since light moves

the same speed in every direction, naturally, you get a circle. (Actually, in space it would form a sphere.)

If we follow three crests, they would make three concentric circles. (Say the words, CREST, CREST, CREST as the three circles are emitted. You want the visitors to get used to the rhythm of CREST, CREST, CREST.)

Now let’s put in a “receiver” on the right side of the screen. Watch how the rhythm of the CREST, CREST, CREST leaving the light source is the same as the CREST, CREST, CREST as it hits the receiver.

Now suppose the light source is moving. The crest radiates out from the place where the light source was when it was emitted. What the light source did before that instant, or what it does after, has nothing to do with where the crest radiates from. This is the most important idea to understand in order to understand the Doppler Effect. (Use a laser pointer to point to the tick mark from which the circle radiates.)

If you have three crests, each one radiates from the location that the light source was when that crest was emitted. (Use laser pointer to point to tick mark of third crest.)

What happens when we put the receiver in? In this case the light source is moving toward the receiver. Compare the CREST, CREST, CREST as the light is emitted with the CREST, CREST, CREST as it’s received. The frequency is higher at the receiver than at the source! For sound, the pitch would seem higher. For light, the color would be shifted toward the “blue” or BLUE-SHIFTED.

If the light source were moving away from the receiver, then the CREST, CREST, CREST at the receiver would have a lower frequency. For sound – lower pitch. For light – RED-SHIFTED.

This part shows that two different receivers can observe the very same light source, but, depending upon the motions see different frequencies.

Once again, the circles we’ve been looking at represent the crests of the waves.

And here is a cut-away view of the same thing.4. Buzzer Ball

Spin the buzzer ball again. Have visitors listen for higher pitch when the ball is coming towards them and lower when it is moving away.

Spin ball even faster. Point out that the faster the ball goes, the more the pitch is shifted.5. What does this have to do with astronomy?

At this point the presentation can go one of two ways, ENDING A or ENDING B. A talks about the search for extra-solar planets. The other choice is to talk about the expansion of the universe.

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Pick your favorite or the one most related to events happening in the news. Be sure to keep your audiences interests in mind.

ENDING A - The search for planets around other starsI. Solar Spectrum Chart

Show solar spectrum chart. Point out black lines (gaps) in the spectrum. These are called Fraunhofer (pronounced Frown-hoff-er) lines and are caused by gases in the atmosphere of the Sun which absorb parts of the spectrum generated by the Sun itself. These lines act as markers because specific substances cause specific lines at specific color locations on the spectrum. If, however, the star is moving towards or away from the Earth, the lines will be shifted toward the blue or the red.

I. Star-planet Mobile Show the star-planet mobile (not to scale). Explain that the balance point is called the center of

gravity, and that the planet doesn’t really orbit the star. In fact, both the planet and the star orbit their common center of gravity. (Naturally, the real star and planet don’t hang from a mobile. The center of gravity is located on the imaginary line which connects their two centers. And since the star is much heavier than the planet, the center of gravity is much closer to the star’s center than to the planet’s. In fact, it is almost always within the star, however not at the star’s center.)

Spin the star-planet mobile. Show that the planet causes a wobble in the star. (The heavier the planet compared to the star, the more pronounced the wobble.)

Depending upon how the planet’s orbit is oriented, we can “see” this wobble in the star as light that is red-shifted / blue-shifted / red-shifted / blue-shifted / etc. (This is similar to the raising and lowering of pitch with the Buzzer Ball.)

I. Extra-solar Planets Animation This animation shows a star which is wobbling due to a planet orbiting around it. Point out

how the Fraunhofer lines move towards the blue whenever the star is moving towards the Earth and towards the red when the star is moving away from the Earth.

By looking at the spectrum of light coming from a star, scientists can figure out how the star must be wobbling. (At least the minimum that the star is wobbling. Depending upon the orbit of the planet’s orientation, it is possible that the star is actually wobbling more than the spectrum would suggest.)

I. 51-Pegasus Data Here is actual data of light coming from the star 51-Pegasus, a star that is 48 light-years from

Earth. This indicates a planet at least half the size of Jupiter orbiting at a distance of about five million

miles from the star.

ENDING B - The Expanding UniverseI. Solar Spectrum Chart

Show solar spectrum chart. Point out black lines (gaps) in the spectrum. These are called Fraunhofer lines and are caused by gases in the atmosphere of the Sun which absorb parts of the spectrum generated by the Sun itself. These lines act as markers because specific substances cause specific lines at specific color locations on the spectrum. If, however, the star is moving towards or away from the Earth, the lines will be shifted towards the blue or the red.

Point out the H and K lines. These are produced by calcium. Because they are particularly strong lines, they are relatively easy to pick out in the spectra of stars.

II. Galaxy Chart

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Remind visitors that the faster that an object is moving towards or away from us, the more it is blue- or red-shifted. So scientists can calculate how fast an object is moving towards or away by the amount of Doppler shift.

In 1919, Edwin Hubble was studying the spectra of distant galaxies and discovered that the spectra were all shifted toward the red. That means that they were all moving away from us. Not only that, but the further the galaxies seemed to be, the faster they were speeding away.

Show the positions of the H+K lines of several galaxies. The arrows show how much these lines have been red-shifted away from their normal positions. This allows scientists to calculate how fast each of these galaxies is speeding away from Earth.

Be sure to demonstrate how fast these galaxies are moving in real world terms. For example, Virgo is moving at 2.7 million mph, which is like going from Denver to Grand Junction in 1/2 second. (Snap your fingers to demonstrate 1/2 second.) Boötes is moving at 87.9 million mph, which is like going around the Earth in 1 second!.

What did Hubble make of the fact that the further away a galaxy is, the faster it is moving away from us?

III. Expanding Universe Diagram (with transparency) Show the 100% Expanding Universe Diagram (on opaque paper). Each dot represents a galaxy. This chart is just an illustration and is NOT a real map of the

positions of real galaxies. The entire Milky Way would be inside the little circle, and Earth is deep inside that. Show the 105% chart (on clear transparency material). It is exactly the same, but blown up on

a Xerox machine to 105%. Line the two charts up. Be sure that the circles overlap. It looks like everything is moving

away from us and the further things are from us the faster they seem to be moving away. This is exactly what Hubble observed. He concluded that the Universe must be expanding. Why are WE at the center? Why does everything seem to be expanding away from us? Hubble

concluded that this did not make sense – there is no reason that Earth should just happen to be in the center of the Universe. He realized that no matter where you are in the Universe, it would seem that the entire Universe is moving away from you.

Demonstrate this with one or two other galaxies on the Expanding Universe Diagrams.

Operating Tips Be sure to put your hand through the cord loop on the Buzzer Ball to ensure against accidentally

letting it go.

Questions and AnswersWhat is Doppler Radar?Doppler radar is used by meteorologists for observing the speed and direction of winds within storm clouds, by police to check the speed of cars, and to measure the speed of baseball pitches. In both cases, the main idea is the same. A short wave radio-wave (a form of energy related to light) of known frequency is bounced off the rain droplets (or your car). The radio waves bounce back to the radar receiver and their frequency is measured. If the droplets of rain (or your car) are moving toward the radar, the frequency will be shifted higher. The amount of frequency shift indicates the speed of the droplets (or car) in the direction of the radar.

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Are all galaxies red-shifted?No. The overall expansion of the universe shows up only at great distances. Some galaxies that are close to the Milky Way actually move toward us and are blue-shifted. However, all galaxies beyond a certain distance are red-shifted.

Is it possible to see any planets orbiting other stars?As of the time of this writing (August 2002) no planets have been directly observed. Most extra-solar planets have been discovered by the Doppler method described above. And only large planets, the order of Jupiter, at that. There is some evidence of a very few planets eclipsing their stars. This could only happen if the tilt of that planet’s orbit around its star exactly lines up with the direction of Earth. The eclipsing is observed as a dimming of the light coming from the star. To check the very latest NASA mission plans and Earth-based projects see the great website ORIGINS (http://origins.jpl.nasa.gov/). More than 100 extra-solar planets (outside of our solar system) have been detected either by small perturbations found in the motion of the central star or by the very slight dimming of the star when the planet passes in front of, and blocks a little bit of light from, the central star. It is hoped that telescopes will soon be able to image planets, though this is a huge challenge because planets do not glow as stars do, but shine like our moon by reflected light.

Other Cool Stuff to Try

The planet mobile is an interesting tool in itself. It can be used to illustrate the idea that when one object orbits another, each object affects the other. Each object orbits around their common center of mass. The larger the mass of the central body, the less its motion changes as it satellite orbits it. Demo to try: Get one larger visitor to play the sun and another smaller visitor to play a planet in orbit around it. Ask them to imagine they are standing on opposite ends of a plank that tips like a see-saw.

Fast Facts

You can hear the Doppler effect when you listen to the whistle of a passing train, or a fire engine siren, as the vehicle passes.

Potential Problems

Getting the demo steps out of order. The buzzer ball needs to be first, then the mobile and charts. Be sure to use the wrist loop when using the buzzer ball to ensure that it doesn’t fly off and hit

someone. The jump in topic from sound to light, though both are waves, will lose some people. Be sure to

build the bridge of understanding from sound to light, and not just breeze through this teaching element.

Though the planetary motion mobile is not heavy, the angle at which the device is held makes it seem heavy. A hook is provided in the ceiling to hold the mobile’s weight. Be sure to locate the hook in the ceiling before you start the demo.

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Background materials (websites, videos, articles, digital collections links)

http://archive.ncsa.uiuc.edu/Cyberia/Bima/doppler.html - The Doppler Effect in Astronomy http://www.explorescience.com/activities/Activity_page.cfm?ActivityID=45 - Media clip on

Explore Science.com website http://planetquest.jpl.nasa.gov – NASA website on the search for extrasolar planets http://www.suite101.com/welcome.cfm/extrasolar_planets - Website on Search for planets http://origins.jpl.nasa.gov/ - Excellent NASA website on Origins- search for life elsewhere

Self assessment suggestionsAfter doing the DOPPLER EFFECT DEMO several times, complete the checklist and rubric below by highlighting the box that best describes your performance. Have your team leader observe your demo then complete an identical rubric. Discuss your presentation technique with your team leader along the lines of the rubric.

Assessment for DOPPLER EFFECT DEMO DATE_______ PRESENTER________________

A. Checklist of pre-requisite skills

1. Can set up and put away demo props: Buzzer Ball, Doppler Animation, Solar Spectrum Chart, Expanding Universe Chart, Star-Planet Mobile and Laser Pointer2. Can incorporate the media clips seamlessly into your presentation3. Can explain each of the following sets of related points- Ending A The search for planets around other stars, Solar Spectrum Chart, Star-planet Mobile, Extra-Solar planets animation, 51-Pegasus DataEnding B-The Expanding Universe, Solar Spectrum Chart, Galaxy Chart, Expanding Universe Diagram (with transparency)4. Knows the sequence of this demo a) Spin buzzer ball to attract crowd b) Show Buzzer Ball at rest c) Show Animation: d) Illustrate Buzzer Ball in action e) Explain how Doppler effect is applied in astronomy

B. Rubric for DOPPLER EFFECT DEMOQUALITY LEVELS

OK EXCELLENTTRAITS

Knowledge of the science

Can answer visitor questions correctly Can go beyond visitor question and add interesting facts gleaned from various resources

Effectiveness using props

Can make a smooth presentation using each prop at some time during demo

Can do the demo using several different approaches to suit the current audience

Educational strategy

Presents a step-by-step explanation, allowing visitor to digest one concept before going onto another one.

Uses a step-by-step approach and actively insures visitor is ready to move on by asking appropriate questions

Fluency with media

Can boot up and use all specified media bits without delay

Can also use other media resources for enrichment

Presentation style Can gather and engage a crowd. Presentation sparkles. Enthusiasm, humor, connection with audience make this an amusing and informative event.