Let's try some examples. When a rock is tossed into a calm lake, a surface disturbance radiates from the point where the rock broke the water. The leading edge of all of that wave forms a circle, and that circle is the wavefront for that event. It is moving outward at a constant speed in all directions. Note that it's two-dimensional (2D). Want 3D? You got it.
In a burst of chemical energy, a star shell explodes at a fireworks display. The light moves away from the origin in all directions at the same speed - the speed of light. And the 3D surface of this wavefront is a sphere, and it is expands around the origin at the speed of light. Pick an arbitrary distance, say, 1 kilometer. Anyone at a distance of 1 km from the event in any direction will find that the wavefront reaches him at the same instant of time as anyone else in any direction who is that 1 km from the event. Even someone in an airplane that is 1 km away will be on the wavefront for an instant - that same instant as any other observers 1 km away. Note that the sound will arrive later - but it, too, radiates forming a spherical wavefront. Our observers at 1 km distance from the event all experience the arrival of the sound wave at the same time.
[Note: Yes, the speed of light is a constant, and it appears to "slow down" when it moves through air of different densities and different humidities, etc., etc., etc. We'll save the splitting of hairs for a question on microtomes or the like. Same with sound. It's velocity will vary a bit through air because of the differential properties of the medium and over the distance through which it travels in said medium.]
First answer by ID1135278755. Last edit by Quirkyquantummechanic. Contributor trust: 729 [recommend contributor]. Question popularity: 106 [recommend question]
