A wavefront is a line or surface connecting areas in a wave of similar phase. The wavefront is perpendicular to the direction of propagation. When you drop a stone in puddle, the wavefronts are the circular ripples produced. In a three dimensional wave the wavefronts are surfaces, for a point source they would be expanding spheres, for a collimated beam, like a laser, they would be flat planes. The figure below shows a collimated two dimensional wave. The wavefronts, marked W, join the points on the waves that have the same phase. The gap between the wavefronts is the wavelength.

|  _      |  _      |  _      |  _
| / \     | / \     | / \     | / \ 
|/   \    |/   \    |/   \    |/   \
|     \   |     \   |     \   |      --> Direction of travel
|      \_/|      \_/|      \_/|
|  _      |  _      |  _      |  _
| / \     | / \     | / \     | / \ 
|/   \    |/   \    |/   \    |/   \
|     \   |     \   |     \   |      --> Direction of travel
|      \_/|      \_/|      \_/|
|         |         |         |
W         W         W         W

As can be seen, the wavefronts are a useful physical concept as they represent what we actually see when observing wave phenomena. The above example could be the waves on the ocean as they come ashore.

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