1. THE DOPPLER EFFECT

2. When the source of a sound is moving towards you, the pitch sounds higher than that of the source. When the source moves away from you the pitch sounds lower. This is known as the DOPPLER EFFECT.

3. Why does this happen? The frequency of a sound determines the pitch. The wave fronts ahead of a moving source are compressed and more crests reach the observer per second, meaning a higher frequency.

4. The wave fronts behind a moving source are further apart and fewer crests reach the observer per second, meaning a lower frequency. When the frequency of a wave changes, there is an associated change in wavelength. This can be determined from the wave equation: v = f.λ

5. Wave fronts compressed ahead of source. Pitch higher because frequency higher Wave fronts stretched out behind the source. Pitch is lower because the frequency is lower Direction of movement of object

6. A worked example: A car is traveling towards you at 30 m.s -1 sounding its hooter with a frequency of 300Hz. The velocity of sound is 340m.s -1. What is the frequency of the sound that you will hear? In one second, 300 waves are produced. These 300 waves would take up 340m if the car were not moving. The car is moving towards you at 30m in each second.

7. The 300 waves are compressed into 310m (340 – 30). λ = 310/300 = 1,033m From the above the following formulae arise: λ obs = c – v/f f obs = c/ λ obs

8. Ultrasound and the Doppler Effect The blood flow in the heart can be measured by making use of the Doppler effect. An ultrasound probe is used and the direction of flow and relative velocity of the blood flow can be determined by calculating the frequency shift caused by the flow of a sample of blood.

9. RED SHIFT The Doppler Effect is characteristic of all waves – including light. All stars emit white light and stars moving away from the earth will display light with longer wavelengths – the red colours of the spectrum, due to the Doppler Effect. Astronomers have found that all stars exhibit a red shift – are moving away from the earth and from each other.

10. This is evidence that the universe is expanding. Scientists have used this information to develop a theory about the origin of the universe, referred to as the BIG BANG theory. This theory states that all matter and energy in the universe was once confined to a very dense point. This exploded - sending matter energy – is expanding outwards. This outwards movement still goes on.