Lab report

Experiment 3: The Doppler Effect

In this experiment, you will observe the effect of moving sources on wave frequency.

Materials (1) 8 oz. Styrofoam® Cup *Bathtub or Sink *Water	*You Must Provide

Procedure

1. Fill a sink or bathtub with enough water to have room to tap the cup up and down without hitting the bottom of the tub or sink.

2. Hold the Styrofoam® cup upside down and practice tapping it on the surface of the water (about three times per second) to create evenly spaced waves.

3. Tapping the water at the same frequency, move your object in a straight line in one direction in a broad, sweeping motion—not too fast, or you will have a hard time seeing the effects!

4. As you move the object, observe the spacing of the waves extending in front of the object compared to the ones behind it.

5. Try a number of different speeds and tapping frequencies until you can see a spacing difference between the front and back waves.

Post-Lab Questions

1. How did the waves in front of the object appear compared to the waves behind the object? Explain why they were different.

2. What can you conclude about the effect of a moving source on the velocity of the waves in a medium?

3. How does the Doppler effect help explain why a car’s engine sounds different as the car approaches you compared to after it passes you?

4. The Doppler effect is present in light waves as well. Red light has a slower frequency than blue light. What can you hypothesize about the motion of a distant star that appears “red-shifted” to astrophysicists?