When the frequency of a wave is altered due to
relative motion between the object and the
observer
Think: the siren on an ambulance as it drives
past!
A Diagram
Describe the wavelength and thus the
frequency (pitch) in each location.
In Front:
Behind:
Perpendicular:
A Diagram
In Front: The wavelength is
compressed: a higher frequency (pitch) is heard.
Behind: The wavelength is
expanded/rarefacted: a lower frequency (pitch) is heard.
Perpendicular: The object is
neither moving away or towards so the wavelength is not changed, so the
wavelength is not changed and the original frequency is heard.
What is Observed?
Sound: As a car approaches, the
wavefront is compressed and the frequency is increased (pitch
increases). As it passes, the wavefront is expanded, the frequency thus
decreases (pitch decreases).
Light: Doppler for light is also
known as redshift: a star moving towards us has an increased frequency
(bluer light), and a star moving away from us has a decreased frequency
(redder light)
The Equations
To work with the Doppler effect we
will need to be confident using the equations we learned last week!
If you do not have these equations in your book,
copy them down and include the units for each variable.
\[
\begin{aligned}
v &= f\lambda \newline
f &= \frac{1}{T}
\end{aligned}
\]
Pātai: Speeding Tickets
Police can use radar guns to measure the amount of compression or
expansion of the frequency due to the relative motion of the offending
car.
The gun emits waves with \(f=100MHz\). What wavelength is this?
The gun reads \(f=125MHz\) when the car is approaching.
What wavelength is this?
The gun reads \(f=75MHz\) when the car is receding. What
wavelength is this?
