Physics CAN be fun.

When a racing car is speeding towards you the engine noise seems very high pitched but immediately it passes you the pitch (or frequency) of the engine noise drops. This effect is called the Doppler effect after the physicist Christian Doppler who first recognised it. The physics behind the Doppler effect are very simple; any waveform which is emitted at a constant frequency can exhibit the Doppler effect - if an object emitting sound of a constant frequency moves towards an observer the peaks and troughs of the wave "bunch up" in the direction of travel and are spread out behind the source - in other words there is a frequency shift which occurs in the waves - that frequency shift varies depending on the direction and speed of the movement.

A great example of the Doppler effect can be seen if you watch a duck or a swan swimming on a still pond. The waves the swan makes are closely bunched in front of her (it) but much more spread out behind her (it). It makes a good photo…(but not here today).

This simple phenomenon has been used in lots of interesting ways: in Astronomy it has been shown that light from distant star and galaxies is "red-shifted" - i.e. it appears to us at a lower frequency than star-light is when it is emitted - this proves that the universe is expanding. Simple physics proving a big idea.

The photo I show here is from my branch of science - veterinary cardiology. The image shows an ultrasound "pie-slice" picture of a cat's heart - specifically the left ventricle and the exit from it - the aortic root. The aortic valve is seen towards the bottom left of the image. In this image the muscle of the heart is shown as light grey whilst the blood within the heart is shown black but there is a "red flame" which is reaching into the left ventricle from the aortic valve. This is evidence of aortic valve incompetence - i.e. the valve is not closing neatly so that blood is leaking back into the left ventricle whilst the heart is relaxing (in "diastole"). The picture was obtained by sending high frequency pulses of sound into the heart to "bounce" of the red blood cells. Sound coming back to the ultrasound transducer at the same frequency at which it was emitted is coded black by the instrument but sound returning to the instrument at a higher frequency is coded red. In other words the blood leaking back into the heart is exhibiting a Doppler shift which we can appreciate as this colour signal. In this way cardiac ultrasound can help me to make diagnoses.

When I was at school our Physics textbook was called "Physics is Fun." Sadly few parts of the course demonstrated that. I wish we'd had a memorable lesson about the Doppler effect.