How is the Doppler effect different for light compared to sound?

The Doppler effect for light involves changes in frequency and wavelength due to relative motion, but not speed, unlike sound.

The Doppler effect is a phenomenon observed in both sound and light, where the observed frequency and wavelength of a wave changes if the source of the wave and the observer are moving relative to each other. However, the way this effect manifests in light is different from how it manifests in sound due to the fundamental differences between the two.

Sound waves are mechanical waves that require a medium to travel through, such as air or water. The speed of sound is determined by the properties of the medium, such as its temperature and pressure. When the source of a sound wave is moving relative to an observer, the speed of the sound wave changes for the observer. This change in speed results in a change in the observed frequency and wavelength of the sound wave, which is the Doppler effect for sound.

Light waves, on the other hand, are electromagnetic waves that can travel through a vacuum. The speed of light is constant in a vacuum and does not depend on the relative motion of the source and the observer. Therefore, when the source of a light wave is moving relative to an observer, the speed of the light wave does not change for the observer. Instead, the observed frequency and wavelength of the light wave change due to the relative motion. This is the Doppler effect for light.

The Doppler effect for light is often observed in astronomy. When a star is moving away from us, its light appears more red than it actually is, a phenomenon known as redshift. Conversely, when a star is moving towards us, its light appears more blue, known as blueshift. These shifts in colour are due to the Doppler effect changing the observed frequency and wavelength of the star's light.

IB Physics Tutor Summary: The Doppler effect changes how we see light and hear sound when the source moves towards or away from us. For sound, it's about the wave's speed changing due to the medium, affecting its frequency and wavelength. For light, which doesn't need a medium, only its frequency and wavelength change because light's speed stays constant. This difference is crucial in studying stars and galaxies.