Doppler EffectDoppler Effect

HISTORY• The Doppler effect was

named after Christian Doppler, who first came up with the idea in 1842.

• The Doppler effect doesn't just apply to sound. It works with all types of waves, which includes light. Edwin Hubble used the Doppler effect to determine that the universe is expanding.

As a wave source approaches, an observer encounters waves with a higher frequency. As the wave source moves away, an observer encounters waves with a lower frequency. This apparent change in frequency due to the motion of the source (or receiver) is called the Doppler effect.

Definition

The greater the speed of the source, the greater will be the Doppler effect.

Doppler Effect In:• SOUND• LIGHT

Sound

The Doppler effect causes the changing pitch of a siren. When a fire truck approaches, the pitch sounds higher than normal because the sound wave crests arrive more frequently. When the fire truck passes and moves away, you hear a drop in pitch because the wave crests are arriving less frequently.

The change in loudness is not the Doppler Effect! It is the shift in frequency!

• Police use the Doppler effect of radar waves to measure the speeds of cars on the highway. • Radar waves are electromagnetic waves.• Police bounce them off moving cars. A computer built into the radar system compares the frequency of the radar with the frequency of the reflected waves to find the speed of the car.

LightThe Doppler effect also occurs for light.

• When a light source approaches, there is an increase in its measured frequency.

• When it recedes, there is a decrease in its frequency.

Astronomy and the Doppler Shift

• Astronomers can measure the Doppler Shift of a moving object.

• The radio wave has a known frequency for stationary objects.

• If target is moving away, waves in the signal will be stretched, and will have a lower frequency.

• If the target is moving closer, waves will be compressed and will have a higher frequency.

• Increasing frequency is called a blue shift , because the increase is towards the high-frequency, or blue, end of the spectrum. • Decreasing frequency is called a red shift , referring to the low-frequency, or red, end of the color spectrum. • Distant galaxies show a red shift in their light. A measurement of this shift enables astronomers to calculate their speeds of recession.

QUESTION

• When a source moves towards you, do you measure an increase or decrease in wave speed?

Neither! It is the frequency of a wave that undergoes a change, not the wave speed.

CasesCases

Case 7• If the source of sound is moved away from the

observer at rest towards wall, then in this case observer hears two nodes of different frequencies.

• The note heard directly from the receding source is lowered in pitch by the motion.

• The other note is due to the waves reflected from the wall, which is raised in pitch , that is so because the source is moving towards the wall.

• The superposition of these two waves produce beats.

APPLICATIONSAPPLICATIONS

WEATHER STATIONS• Radio waves are emitted from

a weather station at a specific frequency.

• The waves are large enough to interact with clouds and other atmospheric objects.

• The waves strike the objects and bounce back towards the Station.

• If the clouds or precipitation are moving away from the Station, the frequency of the waves reflected back decrease.

• If the clouds or precipitation are moving towards the station, the frequency of the waves reflected back increase.

POLICE RADAR SYSTEMS

A police officer aims his radar gun at an approaching vehicle. The gun sends out a burst of radio waves at a particular frequency. The radio waves strike the vehicle and bounce back toward the radar gun.An electromagnetic wave is emitted by a source at the side of the road attached to a Police Car. The Wave is reflected by a moving vehicle, which thus acts as a moving source. The reflected wave will have a Doppler Shift in frequency. This frequency shift is measured by using the phenomenon of beats, which permits the measurement of the speed of vehicle.

BATS• A bat determines the

location and nature of objects by sending ultrasonic waves.

• The bats are provided with some sensory devices to find speed of the objects in their surrounding by sensing the Doppler frequency shift.

AEROPLANES• Doppler Shift is also

applicable in radar system. The radar is a device which transmits and receives radio waves from the aeroplanes if an aeroplane is coming.

ASTRONOMY• The relativistic Doppler

effect is important in astronomy, if the source is receding at high speed from an observer, then a lower frequency (or longer wavelength) is observed for certain spectral or characteristic frequency.

• This is the origin of the term Redshift, the wavelength of visible light are shifted towards longer wavelength (red).

ANY QUESTIONS???