Hello, young physicists! Let's dive into the exciting world of waves and the Doppler effect. Imagine you're chilling at the beach, and you can see waves crashing onto the shore. You can visualize the Doppler effect by thinking of how waves interact with objects on the shore.

The Doppler effect happens when a wave source (like a car horn or a speaker) and an observer (you) are moving relative to each other. When they move closer or further apart, the frequency of the waves changes. If you've ever heard a car zoom past you with its horn honking, you've experienced the Doppler effect! The horn's pitch changes as it goes by.

Let's get a bit technical. A point source (S) is a tiny object that emits waves. Imagine a pebble dropped in a pond, creating ripples that spread outwards in circles. These circles are called wavefronts. Now imagine this in 3D, and you have spherical wavefronts spreading out in all directions.

Picture a 2D wavefront diagram (Figure 3) with a point source (S) and an observer (O). The red circles represent wavefronts at different times. The wavefronts expand outwards like ripples. When neither S nor O is moving, the wavefronts reach O at regular intervals. So, the source and observer agree on the wave's frequency.