Doppler effect

The Doppler effect is a change in frequency and wavelength of a wave. It is caused by the change in distance between the thing creating the wave (causer) and whatever is measuring seeing or hearing the wave (watcher or observer).

Another word for "causer" is "sender" or "source". Another word for "change in distance" is "speed" or "relative velocity". 
A common example is sitting in a car while another car goes by the watcher.  The watcher will hear a change in pitch, while the sender will not.

For waves read all the waves that can send out or reflected by an object. And for reflection read change of direction of a wave. 

 Effects of changes in distance 
center
If observer and creator of the wave get closer, the frequency is higher and the wavelength is shorter.
 For light, this causes a shift in colour towards the blue end of the spectrum called a blue shift. The faster something is moving toward us, the greater the blue shift.
 For sound, this causes the sound to become higher in pitch

If the distance between the observer and creator gets longer, the frequency is lower and the wavelength is longer.
 For light, this causes a shift towards the red end of the spectrum called a red shift, the faster something is moving away, the greater the red shift.
 For sound, this causes the sound to become lower in pitch.

Light waves can also be read, examples are:
 Microwaves
 Radio waves

An extreme example of the doppler effect is an aeroplane flying at a faster speed than the speed of sound and how the wall of sound is heard on the ground.

 Related pages 
red shift
blue shift

 References 

Category:Doppler effects
Category:Wave mechanics