Physics: The Doppler Effect… (The Dopppppplerrrrrr Efffffecccct)

Get on the same wavelength as this common phenomenon.

Some concepts in physics are hard to get your head around,but this isn’t one of those cases because almost everybody has experienced thatweird thing where an ambulance siren seems to change as the vehicle passes you.Same with motorbike engine noise and someone yodelling on a skateboard. Common,everyday events.

Well, it’s all about waves.

Not the waves of tears you’re crying after listening to thatsong. We’re talking sound and light waves – mainly sound waves here, because it’seasier to use them as examples.

In the case of that ambulance, you’re the observer. And thesound waves produced by its siren change in frequency as the ambulance travelstowards you – and as it travels away from you. As it gets closer, those soundwaves get compressed (which means a higher frequency and pitch). As it getsfurther away, they get stretched out (so a lower frequency and pitch).

It’s all about the relative motion between the source andthe observer, and next time an ambo passes, pay attention to how thesound changes. (Please don’t call 000 because you got impatient and wanted to tryit.)

How can you work it out?

So, for reasons, we want to know exactly what impact the Doppler Effect has in different scenarios. As with almost everything, there’s an equation for that.

And here’s what all the letters mean:

f‘ = observed frequency
f = actual frequency
v = velocity of wave
v= velocity of observer
v = velocity of source

What’s this good for?

Obviously you’d hope this has better uses than explainingwhy things sound different sometimes. And your hope would totally come true,because it turns out the Doppler Effect is a useful trick to know in a wholerange of areas. Here are three:

In robotics, it helps robots navigate environments withmoving obstacles with dynamic real-time path planning.

In astronomy, it’s used to measure which stars and galaxiesand junk are moving towards or away from us, and how fast that’s happening. Inthis case, they use light waves – blue light has a higher frequency and redlight has a lower one, so depending on a star’s colour you can tell whichdirection it’s moving compared to us.

In medicine, it is useful for ultrasounds where you want tolook at stuff like the movement of blood and tissue.