Q. What are the wave fronts?

Answer:-

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

Let's try some instances. When a rock is tossed into a calm lake a surface annoyance 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 external at a constant speed in all directions. Note that it is two-dimensional (2D). Want is 3D? You got it.

In a rupture of chemical energy a star shell explodes at a fireworks display. The light moves apart from the origin in all directions at the same speed - the speed of light. In addition to the 3D surface of this wave front is a sphere and it is expands around the origin at the speed of light. Choose an arbitrary distance say 1 kilometre. Anybody 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 somebody 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 reach later - but it too radiates forming a spherical wave front. Our spectators at 1 km distance from the event all experience the arrival of the sound wave at the same time.