According to Huygens’ Principle, as a wavefront propagates, its shape evolves based on the medium and the geometry of the wavefront:
- In a uniform medium, a wavefront maintains its overall shape while expanding.
- A plane wavefront remains flat and moves forward in parallel layers.
- A spherical wavefront becomes larger and more curved, but at great distances it can begin to resemble a plane wave due to the large radius.
- At boundaries or obstacles, the wavefront can change shape due to reflection, refraction, or diffraction.
- When the wavefront hits a new medium at an angle, it bends (refraction) and changes speed, altering its curvature.
- Near edges or slits, the wavefront spreads (diffraction), and secondary wavelets from the edge create new curved wavefronts in the shadow region.
In essence, Huygens’ Principle shows that the wavefront evolves by forming a new surface from secondary wavelets, preserving or modifying its shape depending on the conditions it encounters.