You may have had the experience when you felt that the siren of an ambulance moving towards you was high-pitched but became low-pitched after the ambulance passed you and started moving away 🚑

→ This is due to Doppler effect.

Doppler Effect

Let's take a look at the situation when the ambulance is moving to the right.

With respect to the person on the right...

- The ambulance is approaching the person.

- The ambulance emits sound with a certain frequency. Since the ambulance is moving to the right, each consecutive siren originates from a position that is closer to the person.

- Each consecutive siren has a shorter distance to travel. It feels like the wavelength of the sound wave is shorter.

- It takes less time for the emitted siren to reach the person on the right.

- The person feels that the frequency of the emitted siren is higher than the actual frequency of the siren.

- Because the person feels that the frequency is higher, he will experience high-pitched siren sound. 

With respect to the person on the left...

- The ambulance is moving away from the person.

- The ambulance emits sound with a certain frequency. Since the ambulance is moving to the right, each consecutive siren originates from a position that is further away from the person.

- Each consecutive siren has a longer distance to travel. It feels like the wavelength of the sound wave is longer.

- It takes more time for the emitted siren to reach the person on the right.

- The person feels that the frequency of the emitted siren is lower than the actual frequency of the siren.

- Because the person feels that the frequency is lower, he will experience low-pitched siren sound. 

In IGCSE Physics, you should be able to identify how the wavelength and frequency changes to an observer when there is a moving source of sound.

The Doppler effect can also be observed in lights 💡

When a light source is moving towards the observer...

- It will seem as if the wavelength shortens and the frequency increases.

- Because the observed wavelength is shortened, there will be a blue-shift of the light wave.

When a light source is moving away from the observer...

- It will seem as if the wavelength expands and the frequency decreases.

- Because the observed wavelength is lengthened, there will be a red-shift of the light wave.

The equation linking wavelength, velocity of moving light source, and speed of light:

(λ - λ₀) / λ₀ = v / c

λ = observed wavelength

λ₀ = emitted wavelength from the source

v = velocity of moving light source

c = speed of light

The Doppler effect of light can be related to how the red-shift of emission spectrum of stars acts as an evidence for the Big Bang theory.

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