Page 203 - Physics Coursebook 2015 (A level)

Chapter 13: Waves
8
Figure 13.21 shows the traces of two sound waves displayed on an oscilloscope screen.
9
10
Figure 13.21 For End-of-chapter Question 8.
a State and explain how the wavelengths of the two waves compare. [2] b Explain the meaning of the term phase difference. Illustrate your answer by estimating the
phase difference between the two waves shown on the screen. [3] c Calculate the ratio of the intensities of the two waves. [2]
a State what is meant by the Doppler effect. [2] b A star is moving away from the Earth at a speed of 6.4 × 105 m s−1. Light of frequency 6.500 × 1014 Hz
emitted by the star is received on Earth. Calculate the change in frequency caused by the
Doppler effect. Give your answer to 2 significant figures. [3] c When looking at distant stars and galaxies, the further a star or galaxy is from Earth, the
more the light emitted from the star is shifted towards the red end of the spectrum.
i State and explain whether the stars and galaxies are moving towards or away from the Earth. [2]
ii Explain what the greater shift in frequency for stars that are further away implies. [1]
The Doppler effect can be used to measure the speed of blood. Ultrasound, which is sound of high frequency, is passed from a transmitter into the body, where it reflects off particles in the blood. The shift in frequency is measured by a stationary detector, placed outside the body and close
to the transmitter.
In one patient, particles in the blood are moving at a speed of 30 cm s−1 in a direction directly away from the transmitter. The speed of ultrasound in the body is 1500 cm s−1.
This situation is partly modelled by considering the particles to be emitting sound of frequency 4.000 MHz as they move away from the detector. This sound passes to the detector outside the body and the frequency measured by the detector is not 4.000 MHz.
a i State whether the frequency received by the stationary detector is higher or lower than the
frequency emitted by the moving particles. [1] ii Explain your answer to i. [3]
b Calculate the difference between the frequency emitted by the moving particles and the frequency
measured by the detector. [3]
c Suggest why there is also a frequency difference between the sound received by the particles and
the sound emitted by the transmitter. [1]
191