To understand the Doppler effect

understand the Doppler effect,one must first know something about sound waves.Sound waves are a part of our everyday experience because everything we hear is a sound wave.These waves are produced by objects that are moving or vibrating.The wave itself is simply a disturbance in the air caused by the vibrating object. As an example, consider a simple tuning fork:a tuning fork is designed to vibrate back and forth a certain number of times each second. With each vibration, a sound wave is produced.This is because the tuning fork pushes on the air molecules around it as it moves.When a prong of the fork vibrates forward,it pushes on the air in front of it,forming a small region of compressed air.This compressed air naturally tries to expand.As it expands,it pushes on the air next to it,causing it to compress.This new compressed region then acts on the air next to it,and the process continues so that the compression travels through the air.Meanwhile,the prong of the tuning fork vibrates back in the opposite direction,forming a region of expanded or rarefied air. This rarefaction is a region of low pressure,and the air nearby rushes into it.The rarefaction also travels through the air just as the compression did.As the tuning fork continues to vibrate,alternating regions of compression and expansion travel through the air.This disturbance propagating air disturbance is a sound wave. Not all of us use tuning forks every day,but most of us are familiar with musical instruments and the sound waves they produce.A guitar string is a good example: When a guitar string is plucked,it vibrates back and forth,causing the air molecules next to the string to vibrate back and forth. These vibrating air molecules then bump into adjacent molecules, causing them to vibrate.The disturbance spreads as these molecules in turn vibrate the ones next to them.This disturbance is the sound wave,and one single disturbance is known as a wave front.If unhindered,the wave front spreads outward from the source in an ever expanding sphere,weakening in intensity as it enlarges.With each back and forth motion of the guitar sting, a new wave front is produced and spreads outward,following the one before it. When you speak,vocal chords vibrating in your throat produce sound waves in the same manner as a vibrating guitar string or tuning fork.