Key Areas covered The Doppler effect is observed

Key Areas covered • The Doppler effect is observed in sound and light • The Doppler effect causes shifts in wavelengths of sound and light

What we will do today • State what is meant by the Doppler effect • Describe how the Doppler effect affects sound and carry out an experimental demonstration to show this • Carry out calculations on the above

The Doppler Effect

Doppler Effect: Big Bang Theory Style

What is the Doppler effect? • The Doppler effect is the change in frequency you notice when a source of sound waves is moving relative to you. • When the source moves towards you, more waves reach you per second and the frequency is increased. • If the source moves away from you, fewer waves reach you per second and the frequency is decreased. • Doppler Shift Demonstrator

Moving source • As you can see from the simulation as you move closer to a source, the frequency you, the observer, receives increases. • As you move away from a source, the frequency you receive decreases.

Calculating the frequency • When a source produces a sound of frequency fs, we can calculate the observed frequency, fo, using a formula. • The formula changes slightly depending on whether you move towards the source or move away from the source.

Calculating the frequency Moving towards the source • The observed frequency, fo, is higher: • fo = f s v. (v - vs) • fs = frequency of source • v = speed of sound (approx 340 ms-1) • vs = speed of source • Towards = Take away

Calculating the frequency Moving away from the source • The observed frequency, fo, is lower: • fo = f s v. (v + vs) • Away = Add

Example 1 • What is the frequency heard by a person driving at 15 ms -1 toward a blowing factory whistle (f = 800 hz) if the speed of sound in air is 340 ms-1? • fo = fs v. (v - vs) = 800 340. (340 -15) = 800 x 1. 04 fo = 837 Hz

Example 2 • What frequency would he hear after passing the factory if he continues at the same speed? • fo = fs v. (v + vs) = 800 340. (340+15) = 800 x 0. 931 fo = 766 Hz

2013 Revised Higher C