Download Doppler Effect Demo

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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
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Buzzer Ball
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Doppler
media
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Solar Spectrum Chart
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Expanding Universe Chart
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Star (shown)
Effect
Planet
Demo
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Expanding Universe Charts
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Microphone and PA
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Laser Pointer
Mobile
Main Teaching Points
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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 longterm 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
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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.
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3. Animation
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 – REDSHIFTED.
 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.
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 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. 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 stars
I.
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.
II.
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.)
III.
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.)
VI.
51-Pegasus Data
 Here is actual data of light coming from the star 51-Pegasus, a star that is 48 lightyears 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 Universe
I.
Solar Spectrum Chart
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III.
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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.
Galaxy Chart
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?
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
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Be sure to put your hand through the cord loop on the Buzzer Ball to ensure against
accidentally letting it go.
Questions and Answers
What 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.
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 extrasolar 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.
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Fast Facts
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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
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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
collections links)
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(websites,
videos,
articles,
digital
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 suggestions
After 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
PRESENTER________________
DEMO
DATE_______
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 Pointer
2. Can incorporate the media clips seamlessly into your presentation
3. Can explain each of the following sets of related pointsEnding A The search for planets around other stars, Solar Spectrum Chart, Star-planet Mobile,
Extra-Solar planets animation, 51-Pegasus Data
Ending 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 DEMO
QUALITY
OK
LEVELS 
TRAITS 
EXCELLENT
Knowledge of
the science
Can answer visitor questions
correctly
Effectiveness
using props
Can make a smooth presentation
using each prop at some time
during demo
Presents a step-by-step
explanation, allowing visitor to
digest one concept before going
Educational
strategy
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Can go beyond visitor question and add
interesting facts gleaned from various
resources
Can do the demo using several different
approaches to suit the current audience
Uses a step-by-step approach and actively
insures visitor is ready to move on by asking
appropriate questions
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Fluency with
media
Presentation
style
onto another one.
Can boot up and use all specified
media bits without delay
Can gather and engage a crowd.
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Can also use other media resources for
enrichment
Presentation sparkles. Enthusiasm, humor,
connection with audience make this an
amusing and informative event.