Cart 0

Determining the Motion of Galaxies Using Doppler Redshift

Author: Caitlin M. Matyas

School/Organization:

The Arts Academy at Benjamin Rush

Year: 2016

Seminar: The Evolving Universe

School Subject(s): Physics, Science

The purpose of this curriculum unit is to increase a high school student’s waves, specifically sound and light.  The unit strives to promote critical thinking and discussion in a class that is often mathematically focused.  It allows students to collect authentic data and use that data to draw conclusions about our universe.

After learning basic properties of waves, students will investigate the Doppler effect of sound.  They will then use what they learned to make predictions about what will happen to light waves.  Finally, students will analyze data from distant galaxies in order to describe the motion of our universe.

The hope for the unit is that students will learn to think for themselves and will gain a better understanding of what it is like to work in a research field.

Did you try this unit in your classroom? Give us your feedback here.

Full Unit Text
Rationale

“My students are robots,” is a statement I have made many times over the past few years.  What do I mean by calling them robots?  I started my teaching at a charter school that emphasized the “I do, We do, You do” model.  I would teach them how to solve a certain type of problem; we would practice solving the same type of problem a few times; and then students would solve more problems in the same manner.  If students were able to replicate the method I showed them to solve the problems, they would succeed.  They were never asked to think for themselves or explain an answer.  Students were programmed to answer the questions a certain way and would often forget what they learned after an assessment.

However, I want more for my students.  I want them to be critical thinkers equipped with the skills necessary to solve real problems.  I want them to question theories and laws taught in class; they should want to prove it for themselves through discovery and research, rather than just accepting the concepts taught as fact.  Because of this desire to push my students to higher-order thinking, I have begun to incorporate more inquiry activities and discussions as I encourage my students to make observations, gather and analyze data, and think for themselves.

I will be challenging my students in a few ways throughout this unit.  I will begin the unit with a lecture and activities on the Doppler effect.  They will be asked to apply what they learn about the Doppler effect in order to make predictions about redshifts.  We will have a class discussion comparing the two phenomena.  At the end of the unit they will conduct their own research similar to that of a physicist or astronomer. They will obtain real data and analyze what they collected in order to study the movement of distant galaxies.  I believe these activities will encourage my students to think deeply and apply what they learn.  They will also gain a better understanding of authentic science research through this experience.

Objectives

This unit is designed to be taught to a group of 11th graders enrolled in an introductory physics course.  It will be an addition to the unit on waves already being taught in the course.

The objectives of the unit are the following:

• Describe and calculate the properties of a wave
• Explain the Doppler effect
• Calculate the speed of a source and/or observer using the Doppler effect
• Explain redshift
• Compare the Doppler effect of sound to the Doppler effect of light
• Utilize real data to determine the movement of distant galaxies
• Understand how scientists collect and analyze data in order to make predictions about natural phenomena

Strategies

We will begin the unit with an introduction on waves and their properties.  Students should be able to differentiate between longitudinal and transverse waves.  They should also be able to label and calculate various qualities of a transverse wave, such as the following: crest, trough, amplitude, wavelength, frequency, period, and speed.  Students will then explore the Doppler effect of sound.  They will have both lecture and exploratory labs inside and out of class.  This will then be followed by its relation to the Doppler effect of light.  This will all lead to a culminating activity related to the motion of galaxies.

Students will explore the Doppler effect of sound with an inquiry-based activity using smart phone applications.  One of the most important activities will be a discussion on how the Doppler effect of sound and light are related.  Students will be challenged to predict occurrences based on what they know about the Doppler effect.  Finally, students will choose a galaxy and calculate its distance from Earth using real data.

Classroom Activities

Five lesson plans are outlined below.  This unit is made as an addition to the waves unit already designed for my physics classes.  Depending on your students’ previous knowledge, it may be helpful to spread out the activities over a few more days.  The first lesson especially may need to be broken into smaller lessons if your students have not been previously exposed to wave properties.

 Lesson Plan 1: Properties of Waves Learning Objective ●       Students will be able to analyze known wave properties in order to calculate and describe unknown properties.   Materials ●       Slinky   Key Terms ●       Amplitude ●       Frequency ●       Period ●       Wavelength ●       Crest ●       Trough   Do Now What do you already know about waves?   Procedures 1.      The teacher should begin with a lecture on the properties of waves.  Definitions of the key terms should be explained.  Both written definitions and pictures of waves should be used to ensure understanding. 2.      The teacher can then use a slinky to demonstrate the difference between longitudinal and transverse waves.  A student volunteer can be used to hold one end of the stretched slinky.  The teacher can apply forces in various directions to produce both longitudinal and transverse waves. 3.      Finally, the teacher should demonstrate how to solve problems related to wave properties. 4.      The teacher and students should participate in guided practice of various wave property questions and problems. 5.      The lesson should end with independent practice on wave property questions and problems.   Assessment Exit ticket: A sound wave traveling in air has a wavelength of 0.08m.  Calculate its frequency.   Homework The teacher should assign practice questions and problems on wave properties.   Rationale This lesson sets the groundwork for the rest of the unit.  It is important that students understand wave basics and how different wave characteristics are related to each other.  It may be necessary to break this lesson into a few smaller lessons if your students do not have any background in waves.

 Lesson Plan 2: The Doppler Effect of Sound Learning Objective ●       Students will be able to investigate the Doppler effect of sound in various scenarios in order to calculate and analyze the perceived frequency of a sound.   Materials ●       Video   Key Terms ●       Doppler shift ●       Source ●       Observer   Do Now How does an ambulance or police siren sound as it approaches and then passes you?  Hypothesize why you think this happens.   Procedures 1.      The teacher should begin with a sharing of student answers to the Do Now question. 2.      The teacher should then show a video on the Doppler effect of sound. 3.      Finally, the teacher should lecture on the Doppler effect and demonstrate how to solve problems related to the Doppler shift of sound. 4.      The teacher should then guide students through solving more problems related to the Doppler shift, gradually releasing more responsibility to the students.  The teacher may even allow some time for students to explain their methods of solving each problem to their peers. 5.      Students should be given time to solve problems on their own before the end of class to ensure they fully understand the Doppler effect of sound.   Assessment Exit Ticket: An ambulance emits a sound with frequency of 350 Hz.  What frequency do you hear standing on the sidewalk as the ambulance moves towards you at 15 m/s?  As it moves away from you at 15 m/s?   Homework Doppler Effect Lab (see appendix) This is a great lab that demonstrates the Doppler effect.  Students can see and hear the frequency/pitch change in real time.  This activity may also be completed in class.  However, any outside noise affects the readings, so student groups will need to spread out throughout the school.  Depending on the age, maturity and responsibility of your students, this may not be feasible.  It will also be helpful to demonstrate with a student exactly what they should be doing in the lab to make sure they understand how to complete it.   Rationale This activity introduces the Doppler effect.  Students should be familiar with the idea, although they may not have known the name for the phenomenon.  You may want to reference the sound of cars driving by or ambulance sirens, etc.  As for the video, there are a few free videos on the internet that have great graphics that explain the perceived change in frequency.  This may also be replaced with the student resource created by McGraw Hill.  Finally, it is also important to practice solving problems, as some students may struggle with the algebra when completing the lab activity.

 Lesson Plan 3: Doppler Shift of Light Learning Objective ●       Students will be able to connect what they have learned about the Doppler Effect of Sound to the Doppler Effect of Light in order to hypothesize how light is shifted.   Key Terms ●       Redshift ●       Blueshift ●       Electromagnetic spectrum   Do Now A police siren emits a sound at a frequency of 300 Hz.  After it passes you standing on a sidewalk, you measure a frequency of 280 Hz.  How fast was the police car moving?   Procedure 1.      The teacher should begin with a sharing of student answers to the Do Now question. 2.      The class should then have a discussion about the lab results.  Topics to cover should include the following: a.       Experimental Results b.      Theoretical Results c.       Difficulties in completing the lab d.      Sources of Error e.       Improving the Lab / Possible Supplemental Lab Activities 3.      The teacher should then transition to a lecture on the Doppler shift of light.  The teacher can begin by stating that light and sound behave in similar ways. The following question should be posed to the class: If light behaves similarly to sound, how could astronomers use light from a distant galaxy to determine the motion of the galaxy? The teacher should solicit various responses. 4.      Finally, the teacher should lecture on the Doppler effect and demonstrate how to solve problems related to the Doppler shift of light.  The electromagnetic spectrum should be used, especially to explain the relevance of the names “redshift” and “blueshift”. 5.      The teacher should then guide students through solving more problems related to the Doppler shift, gradually releasing more responsibility to the students.  The teacher may even allow some time for students to explain their methods of solving each problem to their peers. 6.      Students should be given time to solve problems on their own before the end of class to ensure they fully understand the Doppler effect of sound.   Homework Doppler Shift Practice Problems (Sound and Light)   Rationale The class discussion should help students understand the results from the lab and to solidify their understanding of the Doppler effect of sound.  It is also important to understand sources of error and how these could be reduced or eliminated in future trials.  As the discussion transitions onto light, students should be given ample time to brainstorm ideas with their peers.  Finally, the teacher and students should complete practice problems on the Doppler shift of light.  This should give students plenty of practice with the algebra involved with solving Doppler effect problems.

Annotated Bibliography

Tennyson, Jonathan. Astronomical Spectroscopy: An Introduction to the Atomic and Molecular Physics of Astronomical Spectra. London: Imperial College, 2005. Print.

– This book gives a good explanation of spectroscopy and how it is used to make astronomical measurements, including a brief description of Doppler redshift.

Teacher Resources

“A visual demo of the Doppler effect.” Papacosta, Pangratios, The Physics Teacher, 48, 420-420 (2010).

– This article explains a quick demo that teachers can use in class to explain the Doppler effect of sound.

“A different viewpoint on Doppler-effect calculations.” Hoyt, Daniel, The Physics Teacher, 40, 14-16 (2002).

-This article explains the calculations involved in the Doppler effect equation and how common sense can be used to determine sign conventions.

Student Resources

PhET: Free Online Physics Simulations.  < https://phet.colorado.edu/ >

– This website offers many simulations that can be applied to various physics lessons.  Each simulation also includes handouts created by teachers that can be downloaded and modified to fit your objectives.  The simulations named “Sound” is especially relevant to this unit.

– This simulation allows students to control whether the source or the observer are moving.  It shows the wave changing in real time as the source and/or observer move.  It also includes an exercise tab that walks students through the activity if the teacher does not want to make their own.

Appendix

Appendix I: Doppler Effect Lab

Name: _________________________________

Purpose: Students will be able to investigate the Doppler effect of sound in order to calculate the speed of the source and observer.

Materials: smartphones with ability to download applications

Directions:

1. Partner 1, search for and download a “frequency detector” application on your smart phone. The application should be able to detect the frequency of sound. (Partner 1 is the “observer”.)
2. Partner 2, search for and download a “frequency generator” application on your smart phone. The application should allow you to generate sound at a desired frequency.
3. Partner 2, set the frequency to a number between 1500 and 2500 Hz. Turn the volume up as high as it can go.  Partner 2 is the “source”.
4. Trial 1: Partner 1 stands still as Partner 2 sprints towards Partner 1. Make sure Partner 2 moves in a straight path towards partner 1 and runs at a constant speed.  Partner 1 records the frequency detected as Partner 2 is sprinting.
5. Trial 2: Partner 2, now sprints at a constant speed in a straight path away from partner 1 as partner 1 remains stationary. Partner 1 records the frequency detected as partner 2 is sprinting.
6. Trial 3: Now have partner 2 stand still and generate a sound at a certain frequency. Partner 1 sprints at a constant speed in a straight path toward partner 2. Record the frequency detected as you are sprinting toward your partner
7. Partner 1 now sprints at a constant speed in a straight path away from partner 2 as partner 2 remains stationary. Record the frequency detected as partner 1 is sprinting.
8. Complete the necessary calculations and record your data in the table.

Note: It is important that the partner that is moving in each trial is going as fast as they can without endangering themself.  They also need to be moving at a constant speed and try not to swing their arms.  Distance of 10 – 20 meters may work best.

 Trial Source Frequency (Hz) Observed Frequency (Hz) Source Speed (m/s) Observer Speed (m/s) 1 0 2 0 3 0 4 0

What happens to the observed frequency if the source and observer approach each other?

What happens to the observed frequency if the source and observer move away from each other?

The Doppler effect can be described by the following formula:

,

where f’ represents the shifted frequency, f represents the frequency of the source, v is the speed of the wave, vo is the speed of the observer, and vs is the speed of the source.

Use the above formula to calculate the respective speeds for each trial.  Show all work below. (Note: Use 340 m/s as the speed of sound.  Vo is added if moving towards and subtracted if moving away from the source.  vs is added if moving away and subtracted if moving towards the observer.)

Appendix II: Galaxy Shift Lab

Name: ___________________________

Purpose: Students will be able to investigate the Doppler effect of light in order to make observations about the motion of our universe.

Procedure:

1. Go to http://skyserver.sdss.org/dr12/en/tools/chart/navi.aspx.
2. Scroll around the sky until you find a galaxy you would like to investigate.
3. Select the area where the galaxy is located (click on the galaxy). Click “Explore”, located on the right side of the page.
4. If the following conditions are met, you may move on to the next step. Otherwise, you will need to go back to find a new location in the sky.
1. Type: Galaxy (not Star)
2. There are no Warnings on the page.
3. There is an Optical Spectra Chart on the page
5. Click the Optical Spectra Chart to open it.
6. Locate H-alpha. Record its wavelength.
7. Locate H-beta. Record its wavelength.
8. Calculate the fractional shift for each line. Note how they compare.
9. Calculate the velocity of the galaxy.
10. Share your data on the Google doc.

 Line Rest Wavelength Galaxy’s Observed Wavelength (nm) Shift (nm) Fractional Shift Velocity (km/s) H-α 656 nm H-β 486 nm

Show calculations below:

Shift: Difference between observed wavelength and rest wavelength.

Fractional Shift: Shift in Wavelength / Rest Wavelength  (Hint: The answer can be confirmed on the galaxy’s data page)

Velocity: Fractional Shift * Speed of Light

Use the Doppler equation and wave properties to prove that the calculations you did above represent the velocity of the galaxy. (Hint: Use algebraic manipulations and substitution to isolate vs. vo is considered 0 since we are calculating the galaxy’s speed in reference to Earth.)

How do the observed wavelengths compare to the rest wavelengths?  What does that tell you about the motion of the galaxy?

What trends do you notice about the data from the other galaxies uploaded onto the Google docs by your peers?  What can you conclude about the motion of our universe?

Standards

Portions of this unit align to the Pennsylvania Common Core standards and the Next Generation Science Standards shown below.

• PA: 3.2.P.B5: Explain how waves transfer energy without transferring matter. Explain how waves carry information from remote sources that can be detected and interpreted. Describe the causes of wave frequency, speed, and wavelength.

• NGSS: HS-PS4-1. Use mathematical representations to support a claim regarding relationships among the frequency, wavelength, and speed of waves traveling in various media.

• NGSS: HS-ESS1-2. Construct an explanation of the Big Bang theory based on astronomical evidence of light spectra, motion of distant galaxies, and composition of matter in the universe.