Constant Wave Speed: In a given medium, the speed of the wave is a constant property. Therefore, according to the wave equation , any perceived change in the wavelength must result in an inverse change in the frequency to keep the product the same.
Wavefront Compression: When a source moves toward an observer, it emits a wavefront and then moves forward before emitting the next one. This causes the distance between successive wavefronts to decrease, effectively 'squashing' the waves in the direction of motion.
Wavefront Expansion: Conversely, as the source moves away, it travels a certain distance away from the previously emitted wavefront before releasing the next. This increases the physical gap between peaks, resulting in a 'stretched' wave with a longer wavelength.
| Feature | Source Moving Toward Observer | Source Moving Away from Observer |
|---|---|---|
| Wavelength | Appears Shorter (Compressed) | Appears Longer (Stretched) |
| Frequency | Appears Higher | Appears Lower |
| Sound Pitch | Higher Pitch | Lower Pitch |
| Light Shift | Blue-shift (Higher frequency) | Red-shift (Lower frequency) |
| Wave Speed | Remains Constant | Remains Constant |
The Constant Speed Rule: Examiners often set traps by suggesting the wave speed increases as a source approaches. Always state that wave speed is constant in a uniform medium; it is only the wavelength and frequency that change relative to the observer.
Directional Logic: Before starting a calculation, perform a 'sanity check' on the direction of motion. If the object is moving away, your final calculated frequency MUST be lower than the source frequency.
Labeling Diagrams: When drawing the Doppler effect, ensure the concentric circles representing wavefronts are not centered on the same point. The centers should shift in the direction of the source's velocity to show compression.
Misinterpreting Distance: A common error is thinking the Doppler effect depends on how far away the object is. In reality, the effect depends on the velocity of the object, not its proximity; a distant object moving fast has a greater shift than a nearby object moving slowly.
Confusing Loudness with Pitch: Students often confuse the increasing volume of an approaching siren with the Doppler effect. The volume increase is due to proximity (intensity), while the Doppler effect specifically refers to the change in pitch (frequency).
