Doppler Effect Concepts

Introduction to the Doppler Effect

Imagine you’re standing still and there’s a car honking its horn while speeding towards you. As it gets closer, the sound of the horn seems to get higher in pitch. Then, as it zooms past you and moves away, the pitch suddenly drops. What you’ve just experienced is the Doppler effect.

So, what’s happening here? It’s all about the waves. Sound travels in waves, right? When the car is moving towards you, those sound waves get squished together. Because the waves are closer together, they reach your ears more frequently, which we hear as a higher pitch.

But once the car passes you and starts moving away, the opposite happens. The sound waves get stretched out. They hit your ears less frequently now, which we perceive as a lower pitch.

The cool thing is, this isn’t just about sound. The same concept applies to light waves from stars. If a star is moving towards us, its light waves get compressed, and we see the light as slightly bluer. If it’s moving away, the waves stretch out, and the light appears a bit redder.

Sometimes this is a bit hard to visualize so let’s think about an analogy with dodgeballs real quick.

The Dodgeball Analogy

Now imagine you’re in a game of dodgeball. The dodgeballs represent waves coming at you. If the thrower (source of the waves) moves towards you while throwing, the dodgeballs hit you more frequently – this is like an increased observed frequency. Conversely, if the thrower moves away as they throw, the frequency of dodgeballs hitting you decreases – a lower observed frequency. No movement from the thrower? Then there’s no change in the frequency of dodgeballs hitting you. The key takeaway here is that it’s the net movement between you and the source that determines the change in frequency.

Now, let’s add a twist. What if you, as the observer, start moving? If you move towards the thrower, you’ll get hit by dodgeballs more frequently, even if the thrower is stationary. This movement increases the observed frequency. Conversely, if you move away from the thrower, dodgeballs will hit you less frequently, decreasing the observed frequency. And if you don’t move then you are going to get hit with dodgeballs at the same frequency as before.

Most often the MCAT will test the Doppler effect conceptually and the information we discussed above is all you will need to know. The key is to determine whether there is net movement towards, net movement away or no net movement and to know what each signifies as summarized below:

  • Net movement towards = increased observed frequency
  • Net movement away = decreased observed frequency
  • No net movement = no change in observed frequency

Try this question set out to help you apply the concept of net movement and it’s intersection with the Doppler Effect.