The Doppler effect

A student uses a ripple tank to investigate water waves.

Diagram 1 shows the ripple tank when viewed from the side.

A dipper moves up and down to produce waves on the surface of the water in the container.

The waves on the surface of the water are transverse.

State what is meant by a transverse wave.

You may draw a diagram to help your answer.

Diagram 2 shows the surface of the ripple tank, at an instant in time, when viewed from above.

Diagram 2

The dipper produces circular waves on the surface of the water.

1 cm in diagram 2 is equal to 2 cm in the laboratory.

Use the diagram to measure the wavelength of the water waves.

The dipper moves up and down with a frequency of 15 Hz.

Calculate the speed of the waves.

The student then makes the dipper move horizontally to the right at constant speed.

The dipper continues to move up and down with a frequency of 15 Hz.

Diagram 3 shows the surface of the ripple tank, at an instant in time, when viewed from above.

Diagram 3

Explain how the frequency of the waves arriving at point X compares with the frequency at which the dipper moves up and down.

A ball is moving through the air with a speed of 54.8 m / s. The ball has a mass of 159 g.

Calculate the energy in the kinetic store of the ball.

Give your answer to 3 significant figures.

The speed of the ball is measured using radio waves.

Radio waves of frequency 2.90 × 10¹⁰Hz travel towards the ball from a source.

The radio waves then reflect off the ball.

The reflected radio waves change frequency depending on the speed of the ball. This change in frequency is due to the Doppler effect.

The change in frequency can be calculated using this formula.

Show that the change in frequency of the radio waves is approximately 1.1 × 10⁴Hz.

[speed of radio waves = 3.00 × 10⁸m / s]

The change in frequency of the radio waves happens because the ball acts as a new source of radio waves.

The ball is moving away from the original source of radio waves.

Explain the change in frequency of the radio waves when the radio waves reflect off the ball.

This question is about cosmology.

Give two pieces of evidence that support the Big Bang theory.

The galaxy NGC 300 is moving away from the Earth at a speed of 1.44 × 10⁵m/s.

Hydrogen atoms in NGC 300 emit light with a reference wavelength of 4.3405 × 10⁻⁷m.

Calculate the wavelength of this light when it is detected at Earth.

Give your answer to five significant figures.

[speed of light in a vacuum, c = 3.00 × 10⁸m/s]

Another galaxy, Centaurus A, is much further away from the Earth than NGC 300.

Explain how the wavelength of light detected from hydrogen atoms in Centaurus A is different from the wavelength of light detected from hydrogen atoms in NGC 300.

The diagram shows two whales in the sea.

Some species of whale communicate with sound signals and visual signals.

Describe the difference between the motion of particles in a transverse wave and the motion of particles in a longitudinal wave.

You may use a diagram to help your answer.

Whale A makes a sound with a constant frequency. This sound is detected by whale B.

When whale A swims towards whale B, the frequency of the sound detected by whale B increases.

This is known as the Doppler effect.

Explain how the Doppler effect causes the frequency of sound detected by whale B to increase.

Visible light from the Sun reaches whale B.

The wavelength of this light is 5.410^–7m.

Calculate the frequency of this light.

[speed of light = 3.0×10⁸m/s]

This question is about red‑shift.

The diagram shows two identical stars, A and B, orbiting each other.

Light from each star arrives at Earth.

As the stars move in their orbit, the wavelength of the light observed at Earth changes due to the Doppler effect.

Add an X to the diagram to show the position of star B when the light emitted from it shows maximum red-shift when detected on Earth.

Light is received on Earth from a distant galaxy.

The longest wavelength of light arriving at Earth from the galaxy is 561 nm.

A lamp on Earth produces the same light with a wavelength of 550 nm.

Calculate the velocity of the galaxy.

[speed of light = 3.0 × 10⁸m / s]

A car is travelling in a straight line along a road. The car passes a person standing at the side of the road.

Before passing the person, the driver of the car presses the car’s horn. The horn makes a loud sound of constant frequency.

The horn continues to make a sound until after the car has passed the person.

Discuss the differences in the frequencies of the sound heard by

• the driver of the car

• the person at the side of the road

The diagram shows the forces acting on a firework at take-off.

Calculate the magnitude of the resultant force on the firework.

State the formula linking resultant force, mass and acceleration.

The mass of the firework is 160 g.

Calculate the acceleration of the firework.

Explain how the acceleration of the firework changes between take‑off and running out of fuel.

You can assume that the thrust force stays the same as the firework burns the fuel.

The firework makes a sound with constant frequency.

As the firework moves upwards, people on the ground notice that the frequency of the sound they hear changes.

This is called the Doppler effect.

Explain how the Doppler effect causes the observed frequency of sound to change for the people on the ground.

Diagram 1 shows water waves just before they reflect off the side of a stationary boat.

Draw the normal at the point where the direction of travel of the waves meets the side of the boat.

Measure the angle of incidence of the water waves.

angle of incidence =_........ degrees

Complete the diagram to show the wavefronts after they reflect off the side of the boat.

The boat starts to move, creating its own waves on the surface of the water.

Surface water waves are transverse.

Describe the difference between transverse waves and longitudinal waves.

Diagram 2 shows the boat moving towards an observer.

Explain why the frequency of the water waves measured by the observer is larger than the frequency of the water waves created by the boat.

State what is meant by the astronomical term galaxy.

Which of these is a large collection of billions of galaxies?

Hydrogen is an element that can emit visible light waves.

The table shows the wavelength of one of these waves when detected from a laboratory sample of hydrogen and when detected from the hydrogen in two different galaxies.

The detected wavelengths from the galaxies are different from the detected wavelength from the laboratory sample because of the Doppler effect.

Galaxy A and galaxy B are both moving relative to Earth.

Compare the motions of galaxy A and galaxy B relative to Earth.

Use data from the table to calculate the speed of galaxy B relative to Earth.

[speed of light, c = 3.010⁸m/s]

A galaxy moves away from the Earth at a speed of 3.9 10⁴km/s.

Light from the galaxy is emitted with a wavelength of 6.2 10⁻⁷m.

Calculate the change in the wavelength of the light that is received by an observer on the Earth.

Calculate the wavelength of light that is received by an observer on the Earth.

One of the pieces of evidence for the Big Bang theory is the red-shift of galaxies.

Explain how the red-shift of galaxies supports the Big Bang theory.

The diagram shows two students holding a piece of string with a box attached.

The box has a buzzer inside and can move along the string.

The buzzer in the box emits a loud sound of constant frequency.

The box moves away from student A at a constant speed.

Explain why the sound heard by student A has a different frequency to the sound emitted by the buzzer.

This question is about satellites and their orbits.

State a difference between an artificial satellite’s orbit and a planet’s orbit.

State a similarity between an artificial satellite’s orbit and a moon’s orbit.

KALPANA-1 was an artificial satellite used to monitor the weather.

The diagram shows the orbit of the satellite.

KALPANA-1 has an orbital speed of 3.1 km/s and completes one orbit in 24 hours.

Calculate the height of KALPANA-1’s orbit above the Earth’s surface.

[radius of Earth 6400 km]

The Doppler effect occurs when there is relative motion between the source of waves and the observer of the waves.

Explain how the Doppler effect causes a change in the observed frequency of the waves.

Suggest why the radio waves from KALPANA-1 detected on the Earth’s surface are not affected by the Doppler effect.

This question is about using radio waves to track an aeroplane.

aeroplane

Radio waves are emitted from an aerial at an airport, and are then reflected back to the aerial from an aeroplane.

The time taken between emitting the radio waves and receiving the waves back at the aerial is 1.9 milliseconds.

Show that the aeroplane is approximately 300 km away from the aerial.

[speed of radio waves 3.0 × 10⁵ km/s]

As the aeroplane travels away from the airport, it sends a signal to the airport using radio waves with a wavelength of 1.2 m.

When the signal is received at the airport, the wavelength is 1.1 × 10^-6 m longer than when it is emitted by the aeroplane.

Calculate the speed of the aeroplane using the formula

change of wavelength speedof aeroplane wavelength
speedof radio wave

[speed of radio waves = 3.0 × 10⁸ m/s]

A car is moving along a road.

The car has an initial velocity of 26 m / s.

The car then accelerates at 1. 2 m / s² until it reaches a velocity of 35 m / s.

State the formula linking acceleration, change in velocity and time taken.

Calculate the time taken for the car to accelerate to 35 m/s.

A radar speed gun is used to measure the speed of the moving car.

The radar speed gun emits radio waves towards the moving car.

The moving car reflects the radio waves back to a detector on the gun.

The detected radio waves have a different frequency from the emitted radio waves.

This change in frequency is used to measure the speed of the moving car.

Explain this change in frequency when the car is moving towards the radar speed gun.

Diagram 1 shows two identical buzzers connected with springs.

Spring A is connected to a post.

A force acts on the apparatus for a short period of time, pulling both buzzers to the right.

During this time, buzzer A moves 2 cm and buzzer B moves 4 cm.

Explain why buzzer B travels with twice the average speed of buzzer A.

A student connects the microphone to an oscilloscope to measure the frequency of sound from the two buzzers while the force acts on the arrangement.

Explain the difference in the measured frequencies of sound from buzzer A compared to buzzer B.

Diagram 2 shows an oscilloscope trace.

The trace represents the sound wave received by the microphone from buzzer A.

Measure the period of the sound waves from buzzer A.

Give your answer in standard form.

Diagram 3 shows two distant galaxies and the Earth.

Light emitted from a nearby star has a wavelength of 590 nm.

The light from an identical star in galaxy P arrives at the Earth.

The wavelength of the light from the star in galaxy P is 600 nm.

Calculate the change in wavelength for light from the star in galaxy P.

Galaxy Q is twice as far away from the Earth as galaxy P.

Suggest how the speed of galaxy Q relates to the speed of galaxy P.

Suggest a value for the change in wavelength of light from an identical star in galaxy Q when the light arrives at the Earth.

Explain how comparing the speeds of the two galaxies provides evidence for the Big Bang theory.

This question is about sound waves.

Sound is an example of a type of wave.

State the name of this type of wave.

The speed of sound in liquids is greater than the speed of sound in gases.

Describe the arrangement of particles in liquids and in gases.

You may use diagrams to help your answer.

Suggest why the speed of sound in liquids is greater than the speed of sound in gases.

A buzzer is an electrical device that produces sound.

The frequency of sound produced by the buzzer is 1400 Hz.

State the formula linking speed, frequency and wavelength of a wave.

Sound travels at a speed of 340 m/s in air.

Calculate the wavelength of the sound wave from the buzzer.

Two students use this method to investigate sound from a moving source.

Student A connects a piece of string to the buzzer.

She spins round so that the buzzer moves in a horizontal circular path.

● she spins round at a slow speed

● she then spins round at a high speed

Student B stands several metres away from student A.

The sound heard by student A is different to the sound heard by student B.

Discuss the differences in the sounds heard by student A and student B.

A toy produces continuous waves when floating on the surface of a pool of water.

The waves spread out as circular wavefronts.

Diagram 1 shows the wavefronts produced when the toy is not moving, as viewed from above.

Diagram 2 shows the wavefronts produced when the toy is moving across the surface of the pool of water.

Draw an arrow on diagram 2 to show the direction the toy is moving.

Explain how the frequency of the waves at point A is different to the frequency of the waves at point B.