20 points!
name an example when you personally have experienced the doppler effect.

The Doppler effect can be experienced when an ambulance is approaching you. Due to the ambulance moving closer to you, the gap between the wavelengths is decreased, leading to you hearing the siren at a much higher frequency than when the ambulance has passed you! Hope this helps! 

When on the beach, for example,imagine standing in the sea near a beach. As you look out, the waves, viewed from nearly head on have no visible distance between them. 

As a wave reaches and washes over you, you realize you are now standing in a trough a few meres long, before the next wave hits you. 

Turn and face the beach, and watch those two waves head on towards the beach, and the visible distance between them has vanished again. 

You can see (hear/measure, etc) wavelength when you are at right angles to it.