IB Physics Sub-topic C5 Notes

The Doppler effect

In C.5, the primary focus is the Doppler effect. This is the change in frequency of a wave incident on an observer due to the movement of a source or observer. You are expected to understand the Doppler effect for both sound waves and light waves.

Sound waves

For sound waves, two scenarios can occur. First, a sound source moves towards or away from a stationary observer.

1. At position A: As the source moves, a wave is emitted. Since the source is moving, the second wave starts further from the first wave. This decreases the frequency of the wave that A will observe.
2. At position B: As the source moves, a wave is emitted. Since the source is moving, the second wave starts closer to the first wave. This increases the frequency of the wave that B will observe.

Second, an observer moves towards or away from a stationary sound source.

1. At position A: As A moves, a wave is emitted. Since A is moving closer, the second wave is received earlier. This increases the frequency of the wave that A will observe, and the formula for this is:
2. At position B: As B moves, a wave is emitted. Since B is moving further, the second wave is received later. This decreases the frequency of the wave that A will observe, and the formula for this is:

Light waves

However, these formulas only work with sound waves due to their low velocities that can be measured. With light, the change in frequency (Δf) and wavelength (Δλ) due to a moving source or observer are related:

$\frac{\Delta f}{f} = \frac{\Delta \lambda}{\lambda}$

When an observer or source is moving away and the frequency of light decreases, it is called a red shift. When an observer or source is moving closer and the frequency of light increases, it is called a blue shift.