2. Doppler Effect

3. The Doppler Effect is the change in the observed frequency caused by the motion of the wave source.

4. Click to start

5. Key Idea to remember, is that the wave speed doesn’t depend on source speed. (This is not obvious!!) Ball: 10ms -1 CAR: 20ms -1 Ball: 10ms -1 How fast is the ball relative to Emma?

6. Sound waves don’t do the same thing! CAR: 20ms -1 SOUND: 340ms -1 Emma measures the same wave speed whatever direction the car is moving.

7. Stationary Source of Waves What do you notice about the wavelength at any point? What do you notice about the frequency at any point?

8. Moving Source What do you notice about the wavelength at any point? What do you notice about the frequency at any point?

9. Stationary SourceMoving Source Wavelength the same in all directions Longer Wavelength Shorter Wavelength Normal Wavelength

10. When the source is approaching, the perceived wavelength is shorter The wave speed is the same The perceived frequency ishigher

11. When the source is moving away, the perceived wavelength is longer The wave speed isthe same The perceived frequency is lower

12. link to simulation

13. Doppler Equation The wavelength when the source is stationary is The wavelength in front of the source is The source frequency is f The perceived frequency is f’

14. Extension

15. The Doppler Equation is: The denominator is v w -v s if the source is moving towards the observor The denominator is v w -v s if the source is moving away from the observor

16. Accelerating Source If the source accelerates towards you, the source speed gets gradually the wavelength gets gradually So the frequency gets gradually If the source accelerates away fom you, the wavelength gets gradually So the frequency gets gradually shorter higher longer lower higher

17. Sketch a graph of frequency v time for the observer as the car passes Constant velocity frequency time

18. Sketch a graph of frequency v time for the observer as the car passes accelerating frequency time

19. Off line Observer Sketch a graph of frequency v time for Fleur FLEUR

20. Off line Observer What happens if Fleur is off line? FLEUR link to simulation

21. Sketch a graph of frequency v time for Fleur

22. In this equation, v s is the speed the source is approaching, not moving

23. Doppler Duck

24. Red Shift Until about 1940, it was thought that the universe was static. Hot objects emit a continuous spectrum. When this light passes through a Hydrogen gas, you get an absorption spectrum. These are dark lines at certain wavelengths

25. Hydrogen Absorption Spectrum as viewed in Earth Laboratory Hydrogen Absorption Spectrum viewed from a Star What do you notice about the spectrum of light from the star? The same spectrum reaches us, but the absorption lines are red shifted. This means the light has a longer wavelength.

26. What does this show? This shows the galaxies are moving away from us. This shows that the universe is………… EXPANDING STAR EARTH