What is the Doppler effect? Explain, with the aid of labelled diagrams, how the Doppler effect occurs - Leaving Cert Physics - Question 10 - 2014

To explain the Doppler effect, we can use labelled diagrams that show concentric circles emanating from a moving source. As the source approaches the observer, the waves are compressed, resulting in higher frequency (shorter wavelength) sound waves being detected. Conversely, as the source moves away, the waves are stretched, leading to a lower frequency (longer wavelength).

The labelled diagrams should depict:

1. Source moving towards the observer with non-concentric circles (waves).
2. Source moving away from the observer with spaced-out concentric circles, demonstrating decreased frequency.

To find the speed of the ambulance, we can use the Doppler formula: $f' = f \frac{v + v_o}{v - v_s}$ where:
f' = observed frequency (820 Hz)
f = source frequency (750 Hz)
v = speed of sound (assumed to be 343 m/s at 20°C in air)
v_o = speed of the observer (0 m/s, as the observer is stationary)
v_s = speed of the source (the ambulance).

Rearranging the formula, we have: $v_s = v \left( \frac{f}{f'} \right) - v_o$ Substituting the values: $v_s = 343 \left( \frac{750}{820} \right) - 0 = 31.5 m/s$ Therefore, the speed of the ambulance is approximately 29 m/s.

1. Speed cameras used by law enforcement to measure the speed of vehicles.
2. Ultrasound scans in medical diagnostics to monitor blood flow and heart conditions.