1. Doppler Effect
The Doppler effect is the apparent change in the frequency of a wave motion when there is relative motion between the source of the waves and the observer.
The apparent change in frequency f experienced as a result of the Doppler effect is known as the Doppler shift.
The value of the Doppler shift increases as the relative velocity v between the source and the observer increases.
The Doppler effect applies to all forms of waves.

2. Doppler Effect (Moving Source)
Suppose the source moves at a steady velocity vs towards a stationary observer.
The source emits sound wave with frequency f.
From the diagram, we can see that the distance
between crests is shortened such that
Since  = c/f and  = 1/f,
We get  vs

3. Doppler Effect (Moving Observer)
Consider an observer moving with velocity vo toward a stationary source S.
The source emits a sound wave with frequency f and wavelength  = c/f.
The velocity of the sound wave relative to the observer is c + vo. c

4. Doppler Shift
Consider a source moving towards an observer, the Doppler shift f is
If vs<<c, then we get
The above equation also applies to a receding source, with vs taking as negative.

5. Doppler Effect (Moving Reflector) [1]
When a sound wave is reflected from a moving object, the frequency of the reflected wave will be different, because of the Doppler effect, from that of the incident wave.
Consider a sound wave of frequency f is directed toward an object moving with a speed v toward the stationary source. v
Reflecting
surface f f’
Transmitter
Receiver f” f’

6. Doppler Effect (Moving Reflector) [2]
For the outward journey, the source is stationary and the observer (reflecting surface) is moving.
For the return journey the source (reflecting surface) is moving and the observer is stationary.
Combining the two equations gives

7. Summary of the Doppler Effect
The Doppler effect can be summarized qualitatively :
The observed frequency of sound is increased when
the source and observer are approaching each other
and is decreased when they are receding from each
other.
Or mathematically,

8. Applications of Doppler Effect (Medicine)
Ultrasonic waves reflected from red blood cells
can be used to determine the velocity of blood flow.
Reflection of ultrasonic waves can also be used to
detect the movement of the chest of a young fetus
and to monitor its heartbeat.

9. Applications of Doppler Effect (Astronomy)
The velocities of distant galaxies can be determined from
the Doppler shift ( The apparent change in frequency).
Light from such galaxies is shifted toward lower frequencies,
indicating that the galaxies are moving away from us.
This is called the red shift.
Red
shift
Blue
shift

10. Applications of Doppler Effect (Radar Speed Trap)
The police monitor the speeds of vehicles with radar gun. The radar gun send microwaves towards the car. The wave reflected back to the gun have a higher frequency because of the Doppler effect.
The microwave receiver in the radar gun detects the difference in frequency f between the emitted signal and the received signal.
The speed of the car is calculated from this, and displayed automatically on a screen.

11. Sonic Boom (Shock Waves)
If a source of sound moves faster than the speed of sound a shock wave occurs.