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.

Frequency of
emitted microwaves
in kHz
of reflected
microwaves in kHz
27 000 000
27 000 002
27 000 000
27 000 000
27 000 000
26 999 997


(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.

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.

[2 marks]

(Total: 5 marks)