In your own words, describe the doppler effect. be sure to give at least one example of the doppler effect.

The doppler effect explains how our perception of sound changes as the source of sound is moving, even though the pitch and other characteristics of sound remains constant. simply, this effect is about the upwards or downwards shift in frequency of wave due to movement of wave source. when the wave source is approaching the observer, the wave frequency and pitch is perceived to have an upwards shift whereas as the source of wave moves away from observer, the frequency is perceived to be less than the actual. this is because when the source is moving away, the speed of source gets added to the speed of wave. 
one example could be the pitch of an ambulance's siren, coming from a distance. as the ambulance approaches, the pitch of siren appears louder than it actually is and as the ambulance moves away, the pitch diminishes, demonstrating the doppler effect. 

2. they have negligible mass.