Doppler Effect
This is a photo of a cop car with a siren. The siren relates to waves via the Doppler effect. In the Doppler effect, the source of the wave is moving. The waves in front(produced by the siren) are mores pressed together making the Wavelength gets shorter equaling a higher frequency of the sounds of the siren. This explains the higher pitch in sounds when the siren passes by, compared with the lower pitch of sounds when the siren is in front seeing as the waves are more spread out. Another example of the Doppler effect is a bug in a pond. The bug is moving, making the waves it produces in front of it closer together then waves behind it.