Doppler Effect Sonar Lesson

Open Ended

​What happens if we place a droplet of water on the still water?

Wavefronts -

​imaginary surface that represents the locus of all the points in an optical field having the same phase at a given instant.

Locus - particular position or place where something occurs or is situated

Wavefronts

A wavefront is dependent on the shape of the source of disturbance

A wavefront does not travel backwards.

Single point source - spherical wavefronts

Wavefronts

Open Ended

On the diagram which part of the diagram represents the wavefronts?

What is the distance between 2 wavefronts?

Distance between 2 wavefronts = 1 wavelength

Spherical Wavefronts

Open Ended

A musician is playing his saxophone. Describe what is heard by the 3 different observers.

When the observer and source of sound are both stationary, the number of waves per second reaching the observer will be the same frequency as the source.

Spherical Wavefronts

What happens to the wavefronts when the point source moves?

Discuss

Moving Point Source

uThe frequency change due to the relative motion between a source of sound or light and an observer is known as Doppler effect.

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Doppler Effect

State the difference between the 2 wavefronts.

​

Doppler Effect (approaching)

When the source moves towards the observer, the effect is to shorten the wavelength of the waves reaching the observer.

Doppler Effect (approaching)

Let v be the speed of sound in air.

A source of sound has frequency f and wavelength λ.

Doppler Effect (approaching)

The source moves towards the observer with speed v_s.

The period of oscillation of the source of sound is T=1/f

Doppler Effect (approaching)

In the time of one oscillation, the source moves towards the observer a distance v_s T

Hence the wavelength is shortened by this distance.

The wavelength of the sound received by the observer is λ-v_s T

Doppler Effect (approaching)

Doppler Effect (approaching)

Doppler Effect (receding)

​The source is moving away from a stationary observer.

The observed length would longer.

Doppler Effect (receding)

In the time of one oscillation, the source moves towards the observer a distance v_s T

Hence the wavelength is longer by this distance.

The wavelength of the sound received by the observer is λ+ v_s T

Doppler Effect (receding)

Doppler Effect

​The frequency is increased when the source moves towards the observer and the frequency is decreased when the source moves away from the observer. 

The above expressions apply only when the source of the waves is sound.

Doppler Effect

​However, a change of frequency (Doppler Shift) is observed with all waves, including light.

In astronomy, the wavelength tends to be measured rather than the frequency.

Doppler Effect

​However, a change of frequency (Doppler Shift) is observed with all waves, including light.

In astronomy, the wavelength tends to be measured rather than the frequency.

Doppler Effect

Edwin Hubble

If the measured wavelength of an observed spectral line is less than that measured for a stationary source, then the distance between the source(star) and detector is decreasing, i.e. moving towards (blue shift).

Doppler Effect

If the measured wavelength is greater than the value of a stationary source, then the source is moving away from the detector (red shift).

Doppler Effect

Doppler Effect

uThe blue and red shift are referred to in this way as the red shift has the longest wavelength in the visible spectrum and blue the shortest.

Multiple Choice

What is the Doppler Effect?

• A change in frequency due to the relative motion of a wave source and an observer

• When a wave overlaps with another wave

• When a wave bounces off of another object

• When a wave bends due to traveling through a different medium

Multiple Choice

the doppler effect causes a shift in...

• pitch

• wavelength

• frequency

• All of the above

Multiple Choice

The pitch of a sound wave relates to the wave's _________________.

• Amplitude

• Frequency

• Wavelength

• Period

Multiple Choice

An ambulance is driving towards you as you stand on the sidewalk. What happens to the pitch of the siren as it comes towards you?

• It increases

• It decreases

Multiple Choice

You are driving in a car and honk the horn at a person standing in the street. As you approach the person, what happens to the pitch of the horn according to you in the car?

• It increases

• It decreases

• It stays the same because you are in the car moving with the sound.

Multiple Choice

You are standing on the side of the road when a police car races past you with its siren on. After the police car passes you, what happens to the pitch of the siren?

• It increases

• It decreases

• It turns red

Multiple Choice

Look at the picture. Who is experiencing a higher pitched sound?

• Observer A

• Observer B

• There's no difference

Multiple Choice

Light can change color due to the Doppler Effect.

• True

• False

Multiple Choice

An astronomer looks at a star in the night sky. Based on his calculations the star's light is shifting towards the color blue. What does this mean about the star?

• It's moving towards the astronomer

• It's moving away from the astronomer

• It's not moving at all

Multiple Choice

when a noise is moving towards an observer, the closer it comes...

• the lower the pitch

• the longer the noise

• the higher the pitch

• the quieter the noise

Multiple Choice

Look at the picture. Who is experiencing a lower pitched sound?

• Observer A

• Observer B

• There is no difference

Multiple Choice

How does the Big Bang Theory explain observations of the Doppler effect in the universe?

• Galaxies make lots of sound waves. 

• Galaxies are blue shifted and moving toward us.

• Galaxies are redshifted and moving away from us. 

• No connection. Light waves aren't visible in the universe. 

Multiple Choice

The Doppler effect applies to light waves as well as sound waves. Astronomers use this to measure the motion of stars and galaxies. In 1929, Edwin Hubble discovered that light from distant galaxies was shifted away from the blue end of the spectrum and towards the red end of the spectrum. Red light has a longer wavelength (and a lower frequency) than blue light. What could Hubble conclude about these galaxies?

• The distant galaxies are growing smaller.

• The distant galaxies are moving towards the Earth.

• The distant galaxies are moving away from earth

Multiple Choice

The car below emits sound waves, represented by red circles, that are heard by an observer. Based on the pattern of sound waves, what is the direction and speed of the car?

• toward the observer, faster than the speed of sound

• toward the observer, slower than the speed of sound

• away from the observer, faster than the speed of sound

• away from the observer, slower than the speed of sound

Multiple Choice

Assuming the speed of sound is 340 m/s, what is the most likely speed of the jet shown below?

• 200 m/s

• 500 m/s

• 100 m/s

• 340 m/s