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.
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.