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.

ii. Explain your answer to part i.

b. Calculate the difference between the frequency emitted by the moving particles and the frequency measured by the detector.

c. Suggest why there is also a frequency difference between the sound received by the particles and the sound emitted by the transmitter.