Red Shift, Blue Shift

Description: A whiffle ball containing a battery operated buzzer is twirled in a circle to demonstrate the Doppler effect. This same effect causes starlight to shift to the blue or red end of the spec-trum if a star is moving towards or away from us.

Objective: To demonstrate how stellar spectra can be used to measure a star's motion relative to Earth along the line of sight.


Materials:

Plastic whiffle ball (12-15 cm in diameter)
Microswitch*
Small buzzer*
9 volt battery*
9 volt battery clip*
Cord (3 meters long)
Solder and soldering iron
Sharp knife or hacksaw blade
Masking tape
*See note about electronic parts.

Procedure:

1. Splice and solder the buzzer, battery clip, and microswitch in a simple series circuit. See the wiring diagram on page 42. Be sure to test the circuit before soldering. Many small buzzers require the electric current to flow in one direction and will not work if the current flows in the other direction.

2. Split the whiffle ball in half along the seam with the knife or saw blade.

3. Remove the nut from the microswitch and insert the threaded shaft through one of the holes as shown in the diagram. If a hole is not present in the location shown, use a drill to make one the correct diameter. Place the nut back over the threaded shaft on the microswitch and tighten.

4. Join the two halves of the ball together with the switch, buzzer, and battery

inside. Tape the halves securely together.

5. Tie one end of the cord to the ball as shown.

6. Station students in a circle about 6 meters in diameter. Stand in the middle of the students, turn on the buzzer, and twirl the ball in a circle. Play out two to three meters of cord.

7. Ask the students to describe what they hear as the ball moves towards and away from them.

8. Let different students try twirling the ball. Ask them to describe what they hear. As an alternate suggestion to the wiffle ball, cut a cavity inside a foam rubber ball and insert the battery and buzzer. The ball can then be tossed from student to student while demonstrating the Doppler effect.

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Last modified prior to September, 2000 by the Windows Team

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