GCSE Questions: Waves
Q2. A radar gun can be used to measure the speed of a car.
Microwaves are emitted by the radar gun and reflected by the car, as shown in the diagram.
(a) The microwaves reflected by the moving car have a different frequency from the microwaves emitted by the radar gun.
What is the name of the effect causing this change in frequency?
Doppler effect 
[1 mark]
(b) The data in the table below are measurements taken from three different cars on the same piece of road.
Car |
Frequency of
emitted microwaves
in kHz |
Frequency
of reflected
microwaves in kHz |
A |
27 000 000 |
27 000 002 |
B |
27 000 000 |
27 000 000 |
C |
27 000 000 |
26 999 997 |
(b)
(i) State which car in in the table is moving towards the radar gun. Give a reason for your answer.
Car A is moving toward the radar gun. The frequency of the reflected waves of an object moving towards the wave source is higher than the emitted waves.
[2 marks]
(ii) State which car in the table is moving the fastest. Give a reason for your answer.
Car C is moving the fastest as it has the maximum change in fequency for reflected waves.
[2 marks]
(Total: 5 marks)
About this site
Car C a change in frequency of 3 kHz whereas car A only has a change of 2 kHz than the emitted waves and car B has no change.
