The Doppler effect is a phenomenon that occurs when there is relative motion between a source of waves (sound or light) and an observer. The key difference between the Doppler effect in sound and light lies in the nature of the waves and the medium through which they propagate.
In the case of sound, it is a mechanical wave that requires a medium, such as air or water, to travel. When a source of sound and an observer are in relative motion, the Doppler effect causes a shift in the frequency of the sound waves perceived by the observer. If the source and observer are approaching each other, the perceived frequency increases (higher pitch), while if they are moving away from each other, the perceived frequency decreases (lower pitch). This is commonly experienced, for example, when a moving vehicle with a siren passes by, and the pitch of the siren changes as it approaches and then moves away.
On the other hand, light is an electromagnetic wave that can travel through a vacuum. The Doppler effect in light occurs due to the relative motion between a light source and an observer. However, unlike sound, the effect of the Doppler shift on light is related to the change in wavelength rather than frequency. When a light source and an observer are in relative motion, if they are approaching each other, the perceived wavelength of the light becomes shorter (blue shift), while if they are moving away from each other, the perceived wavelength becomes longer (red shift). This phenomenon is widely used in astronomy to measure the motion of celestial objects by analyzing the shift in the spectral lines of their emitted light.
In summary, the key distinction between the Doppler effect in sound and light lies in the nature of the waves involved and the characteristics of their propagation. In sound, it affects the perceived frequency (pitch), while in light, it affects the perceived wavelength (color).