The Doppler Effect I Oxford Open Learning

# The Doppler Effect

The Doppler effect is likely something you’ve experienced, even if you’ve never heard of it. Perhaps you noticed the sirens of an ambulance change pitch as it passed you by. The Doppler Effect was named after the Austrian physicist Christian Doppler, who first described the phenomenon in 1842 and is most widely associated with sound waves although the principle applies to all wave forms such as light and radio waves.

It has played a significant part in many scientific areas such as astronomy and medicine and has even been used as evidence for explaining the big bang theory and expansion of the universe.

### The Physics Behind The Doppler Effect

All forms of waves have a particular frequency, which is the number of waves reaching an observer over a given time. It’s this frequency that determines the pitch of a sound. In technical terms, this effect happens because of the perceived alteration in frequency and wavelength of a wave, brought about by the relative motion between the wave source and the observer.

Taking the case of sound waves from an ambulance moving towards you, waves emitted in front of the vehicle are compressed, increasing the frequency which in turn increases the pitch of the sound. As the ambulance passes you by the waves expand and the frequency drops, leading to a lower pitch. As the passengers on the ambulance are travelling at the same speed as the sirens, they hear the sound at it’s true, unaltered pitch.

### Applications In Everyday Life

You may have seen police using speed guns to track the velocity of vehicles or the tracking system that measures the speed of a served tennis ball. Both systems work in the same way, using specialised radar equipment to send out a narrow stream of radio waves. Some of these waves bounce back off the ball/vehicle and are received back by the emitter. A computer system then calculates the velocity of the object by working out the time between the pulses sent out and the pulses received.

This same principle has played a revolutionary role in astronomy, where the study of changes in light frequency by celestial objects has helped to determine their speed of travel as well as their chemical compositions. Meteorologists use the Doppler effect to predict future weather patterns and track their changes through Doppler radar systems which analyse shifts in the frequency of electromagnetic pulses.

### The Doppler Effect In Medicine

The Doppler effect has also allowed major advancements within the field of medicine, such as the introduction of ultrasound technology. During ultrasound scans, a small device emits high-frequency sound waves that are far beyond our normal audio range. They reflect off different parts of the body and are picked up by the probe and turned into a visual image. This way, parents can see an image of their unborn baby, while a surgeon can see a visual aid while navigating an internal procedure.

Doctors are also able to measure blood flow through a patient’s blood vessels with the use of a specialised Doppler ultrasound. By focusing the sound waves on the red blood cells, doctors can determine the existence of blood clots, heart defects or blocked arteries.