2. Doppler effect
Source stands still and
emits waves all around it. All
observers experience the
same frequency.
Source travels to the
right. Waves emitted
forward have a
shortened wavelength;
waves emitted behind
have a lengthened
wavelength.
Images: Bauer
3. Moving observer
Stationary source
Obs
The moving observer will detect waves with a shorter
wavelength because it is moving in the opposite
direction to the waves.
Images: Bauer
4. Moving source
Stationary observer
Obs Obs
The green observer will detect waves with a shorter
wavelength because the source is moving towards it.
The red observer will detect longer waves.
Images: Bauer
5. Doppler sound The sound of a moving
car is different depending
on whether the car is
approaching the
observer or receding.
short wavelength =
high frequency =
high-pitched sound
A moving observer also experiences a higher sound if
she is approaching a stationary source. eg an observer
on a train passing a ringing bell.
6. Doppler calculation f’ = frequency detected
f = frequency emitted
v = velocity of sound
uo = velocity of observer
us = velocity of source
When the velocities are in opposite directions, we add them.
A police car siren emits a Speed of sound = 330 m/s
sound of frequency 600 Hz A cyclist approaches at 18
while travelling at 40.0 m/s. m/s a stationary transformer
What does this sound like humming at 50.0 Hz. What
to a stationary observer wavelength of sound does
behind the car? [535 Hz] she measure? [6.26 m]
7. Red shift
The absorption lines
in the spectrum of
stars are distinctive.
If a galaxy is moving
with great speed, we
can measure the shift
of the lines. If it is
approaching, the
frequency is higher
(blue-shift). If the This is the method which
galaxy is receding Edwin Hubble used to establish
(most are), the lines that the universe is expanding.
are red-shifted. Image: sciencephoto.com
8. Calculating red shift
∆f = change in frequency
v = relative speed of source and observer
An approximation of c = speed of light (300 000 km/s)
the Doppler formulas f = original frequency
when c >> v
A star emits light at 456 THz which is detected
at the earth as 416 THz. What is the speed of
the star relative to the Earth? [26 300 km/s]
Infra-red radiation from a police radar gun
bounces off a car travelling at 30 m/s. The car
receives a frequency of 200 THz; what will be the
change in frequency? [20 MHz]
9. Using Doppler shift
The speed of blood
flow can be measured
using Doppler shift of
ultrasound.
In this application, like the
radar speed trap, there is a
double Doppler shift: once
when the wave hits the
object and again when it is
reflected. Image: photolibrary
10. The background photo shows 1.5 million galaxies artificially
coloured according to their Doppler shift. Red is the
greatest shift (thus most distant). The Milky Way stretches
across the middle of the map.
11. a production
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