Medical physics Question:

A wave front is an imaginary surface joining all points in space that are reached at the same instant by a wave propagating through a medium.

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 that entire 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. 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 wave front 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 wave front 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 wave front 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 wave front. Our observers at 1 km distance from the event all experience the arrival of the sound wave at the same time.