What are wave fronts?

(Physics) A wave front is an imaginary surface joining all points in space that are reached at the similar instant by a wave propagating by a medium.

Let's try some instance. 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 that whole wave forms a circle, and that circle is the wave front for that event. It is moving outward at a constant speed in all directions. 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 similar speed - the speed of light. And the 3D surface of this wave front is a sphere, and it is expands around the source at the speed of light. Pick an arbitrary distance, say, 1 kilometre. Anyone at a distance of 1 km from the event in any direction will search that the wave front reaches him at the similar 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 wave front for an instant - that similar instant as any other observers 1 km away. Note that the sound will arrive later - but it, too, radiates forming a spherical wave front. Our observers at 1 km distance from the event all experience the arrival of the sound wave at the similar time.