The Doppler Effect - AP Physics 2

1. Which color shift indicates an object is moving towards the observer?

   Blueshift. Shorter wavelengths appear bluer, indicating approach toward the observer.

2. How does the speed of sound affect the Doppler Effect?

   Higher speed reduces effect. Higher wave speed in the denominator reduces the fractional frequency change.

3. What is the Doppler Effect?

   The change in frequency or wavelength due to relative motion. This fundamental phenomenon occurs when there's relative motion between source and observer.

4. What are the units of frequency in the Doppler Effect?

   Hertz (Hz). Frequency measures cycles per second, the standard unit for wave oscillations.

5. State the formula for the Doppler shift in light.

   $\Delta \lambda = \lambda_o \frac{v}{c}$. For light, the shift is proportional to velocity divided by light speed.

6. How do you calculate the speed of an object using the Doppler Effect?

   Use observed frequency shift. Rearrange the Doppler formula to solve for unknown velocity.

7. What happens to wavelength during a blueshift?

   Wavelength decreases. Higher frequency corresponds to shorter wavelength in electromagnetic waves.

8. How is the Doppler Effect used in medical imaging?

   Measures blood flow velocity. Ultrasound waves reflect off moving blood cells to measure flow rates.

9. What happens to wavelength during a redshift?

   Wavelength increases. Lower frequency corresponds to longer wavelength in electromagnetic waves.

10. What does $v_o$ represent in the Doppler Effect formula?

    Speed of the observer. Positive when observer moves toward source, negative when moving away.

11. If $v_o = v_s$, what is the effect on the observed frequency?

    No effect on frequency. Equal and opposite velocities cancel out, producing no net Doppler shift.

12. How does the Doppler Effect influence the pitch of a car horn?

    Pitch is higher when approaching. Approaching vehicles create higher pitch, receding vehicles create lower pitch.

13. Calculate observed frequency: $f = 500$ Hz, $v = 340$ m/s, $v_o = 10$ m/s, $v_s = 0$ m/s.

    Observed frequency $f' = 515$ Hz. Observer moving toward stationary source: $f' = 500 \times \frac{350}{340} = 515$ Hz.

14. What is the observed frequency when source and observer are stationary?

    Observed frequency equals source frequency. No relative motion means no change in wavefront spacing or frequency.

15. What is a practical application of the Doppler Effect in law enforcement?

    Speed radar guns. Police use Doppler radar to measure vehicle speeds for traffic enforcement.

16. What does $v_s$ represent in the Doppler Effect formula?

    Speed of the source. Positive when source moves toward observer, negative when moving away.

17. What type of wave does the Doppler Effect apply to?

    Sound and light waves. The effect occurs for all wave phenomena including mechanical and electromagnetic waves.

18. How is the Doppler Effect used in astronomy?

    Measures star velocities. Spectral line shifts reveal whether stars are approaching or receding.

19. Calculate observed frequency: $f = 400$ Hz, $v = 340$ m/s, $v_o = 0$ m/s, $v_s = 20$ m/s.

    Observed frequency $f' = 377$ Hz. Source moving away from stationary observer: $f' = 400 \times \frac{340}{360} = 377$ Hz.

20. When does the Doppler Effect produce no change in frequency?

    When source and observer have no relative motion. Zero relative velocity produces no wavefront compression or stretching.

21. What does $\Delta \lambda$ represent in the Doppler shift formula for light?

    Change in wavelength. This represents the difference between observed and original wavelength.

22. What does $v$ represent in the Doppler Effect formula?

    Speed of sound in the medium. This constant depends on the medium's properties and temperature.

23. What effect does the Doppler Effect have on light from approaching galaxies?

    Blueshift. Light from objects moving closer shifts toward shorter, bluer wavelengths.

24. What does $c$ represent in the Doppler shift formula for light?

    Speed of light in vacuum. The universal constant approximately $3.0 \times 10^8$ m/s.

25. Which color shift indicates an object is moving away from the observer?

    Redshift. Longer wavelengths appear redder, indicating recession from the observer.

26. State the formula for the Doppler Effect for sound.

    $f' = f \frac{v + v_o}{v + v_s}$. Observer velocity is positive when moving toward source, source velocity positive when moving toward observer.

27. Identify $f'$ in the Doppler Effect formula.

    Observed frequency. This is the frequency detected by the observer after the Doppler shift occurs.

28. What effect does the Doppler Effect have on light from receding galaxies?

    Redshift. Light from objects moving away shifts toward longer, redder wavelengths.

29. Which direction does frequency shift if observer moves away from the source?

    Frequency decreases. Observer motion away from source stretches received wavefronts.

30. In the Doppler Effect, what happens to frequency as the source approaches?

    Frequency increases. Approaching motion compresses wavefronts, creating higher frequency.

31. In the Doppler Effect, what happens to frequency as the source moves away?

    Frequency decreases. Receding motion stretches wavefronts, creating lower frequency.

32. Identify $f$ in the Doppler Effect formula.

    Source frequency. The original frequency emitted by the source before any Doppler shift.

33. What does $\lambda_o$ represent in the Doppler shift formula for light?

    Observed wavelength. The wavelength measured by the observer after Doppler shifting.

34. Which direction does frequency shift if observer moves towards the source?

    Frequency increases. Observer motion toward source compresses received wavefronts.

35. Does the Doppler Effect occur in a vacuum?

    Yes, for electromagnetic waves. Light waves don't need a medium, so the effect still occurs in space.

36. How does the Doppler Effect affect weather radar?

    Detects motion of raindrops. Radar waves reflect off moving precipitation, showing velocity information.

37. What is the speed of sound in air at room temperature?

    Approximately 343 m/s. This value varies slightly with temperature and humidity conditions.

38. Calculate $f'$ when $f = 600$ Hz, $v = 343$ m/s, $v_o = 5$ m/s, $v_s = 5$ m/s.

    Observed frequency $f' = 600$ Hz. Equal speeds in opposite directions: $f' = 600 \times \frac{348}{348} = 600$ Hz.

39. Which principle explains the Doppler Effect?

    Wavefronts moving relative to an observer. Relative motion changes the spacing between successive wavefronts.

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