Exploring the Doppler Effect With NASA

1. Review the Doppler effect for sound with students:
   • Discuss familiar examples such as the horn of a passing train or a passing emergency vehicle with siren.

     While the sound waves seem to bunch up for the observer as the firetruck approaches, notice that the frequency of the sound waves does not change. Image credit: NASA |

   • Explain that any wave phenomenon seems to "pile up" when moving toward an observer and "stretch out" when receding from the observer. This makes it seem to the observer as if the frequency has changed even though it has not. With sound, that means listeners hear an apparent change in pitch. With visible light, observers see an apparent change in color. With all kinds of light, there's an apparent shift toward the blue or red end of the spectrum. For example, some students may have heard of “red shift” as an indicator of the expansion of the universe.

     A light source moving away from the observer appears "redshifted" while a light source moving toward the observer appears "blueshifted." Image credit: NASA |

• Discuss familiar examples such as the horn of a passing train or a passing emergency vehicle with siren.

  While the sound waves seem to bunch up for the observer as the firetruck approaches, notice that the frequency of the sound waves does not change. Image credit: NASA |

• Explain that any wave phenomenon seems to "pile up" when moving toward an observer and "stretch out" when receding from the observer. This makes it seem to the observer as if the frequency has changed even though it has not. With sound, that means listeners hear an apparent change in pitch. With visible light, observers see an apparent change in color. With all kinds of light, there's an apparent shift toward the blue or red end of the spectrum. For example, some students may have heard of “red shift” as an indicator of the expansion of the universe.

  A light source moving away from the observer appears "redshifted" while a light source moving toward the observer appears "blueshifted." Image credit: NASA |

1. Share that the amount of Doppler shift is proportional to the line-of-sight velocity of the object, and light travel time tells us the distance to a spacecraft. These parameters assist in tracking and locating spacecraft in 3-D space.

   This animation shows how the DSN compares data obtained from multiple antennas to identify a spacecraft's position. Credit: NASA/JPL-Caltech |

1. Have students stand around the perimeter of the room, or take the class outdoors to form a large circle. Throw the Doppler ball to different students around the room, encouraging all students to listen with each throw.

1. Ask students to share what they noticed.

   The amplitude (volume) change is apparent as the ball gets closer or farther, but the Doppler shift can also be very noticeable as the ball goes by students along the line of flight. Those off the line may notice a much diminished effect.

Ask students to share what they noticed.

The amplitude (volume) change is apparent as the ball gets closer or farther, but the Doppler shift can also be very noticeable as the ball goes by students along the line of flight. Those off the line may notice a much diminished effect.

1. Ask students to explain why students in the line of flight notice the Doppler shift more than those off the line of flight.

1. Tie a one-meter length of string securely around the Doppler ball so the ball will not be at risk of coming loose when swung at high speed.

1. Resume the demonstration by orbiting the Doppler ball horizontally above your head, like a lasso, while you stand in the center of the circle formed by students. Ask students if they can hear a Doppler shift. Have them describe what they are hearing.

   Most students with good hearing should be able to discern the shift in this circumstance even if they could not when the ball was being tossed.

Resume the demonstration by orbiting the Doppler ball horizontally above your head, like a lasso, while you stand in the center of the circle formed by students. Ask students if they can hear a Doppler shift. Have them describe what they are hearing.

Most students with good hearing should be able to discern the shift in this circumstance even if they could not when the ball was being tossed.

1. Swing the Doppler ball in a vertical orbit, so that the orbital plane is perpendicular to the floor. Ask who can hear a Doppler shift.

   Students in the plane of flight will be able to hear the Doppler shift, but the greater the angle between students and the plane of flight, the less they’ll be able to hear it. Ask students to hypothesize about why some students can hear it and some students now cannot.

Swing the Doppler ball in a vertical orbit, so that the orbital plane is perpendicular to the floor. Ask who can hear a Doppler shift.

Students in the plane of flight will be able to hear the Doppler shift, but the greater the angle between students and the plane of flight, the less they’ll be able to hear it. Ask students to hypothesize about why some students can hear it and some students now cannot.

1. Remaining in the same location, rotate your body about 30 degrees to the right or left so the vertical orbit plane also shifts by that same angle. Orbit the Doppler ball in the new vertical orbit plane. Ask students who can discern a Doppler shift. Can the same students hear it, or are there different students who can hear it now? Ask students to confirm or revise their previous hypothesis.

1. Repeat the step above several times until students are able to explain that being in the plane of orbit provides the most discernible Doppler shift. Being slightly out of plane provides a less obvious Doppler shift. Being perpendicular to the plane provides no Doppler shift.

1. Explain that scientists can use this planar detection of the Doppler shift to determine a spacecraft's location in 3-D space. The change in frequency is a function of the angle between the line of sight and the line of motion. For trigonometry students, mention that the mathematics of this involves computing the cosine of the tilt angle. The cosine of 90 degrees is zero which explains why no Doppler shift is detectable at a perpendicular angle.