Mahi Tuatahi 🔗
- Work on Textbook 4a
- Hand in your homework!
The Doppler Effect 🔗
- 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?
Converting Units 🔗
- Convert $1000m$ into $Mm$
- Convert $1000m$ into $mm$
- Convert $1Gm$ into $nm$
- Convert $620 \mu m$ into $km$
- Convert $0.012km$ into $nm$
