Wavefronts, diffraction and Huygens' principle

Wavefronts, diffraction and Huygens' principle

A wavefront joins points that are in the same phase. Huygens' principle models every point on a wavefront as a source of secondary wavelets; the next wavefront is the envelope of those wavelets. Diffraction is the spreading of waves when they pass through a gap or around an obstacle. It is most noticeable when the gap or obstacle dimension is similar to the wavelength. A small gap relative to wavelength causes strong spreading; a much larger gap produces less spreading. Two-dimensional diagrams may show circular wavefronts from a point source, while three-dimensional wavefronts are spherical even though a page can show only a cross-section.

Work it through

Water waves approaching a narrow harbour entrance spread into the sheltered area because the gap size is comparable with the wavelength. If the gap is widened greatly while wavelength stays the same, the spreading angle decreases. Use the diagram to distinguish the original straight wavefronts from the curved wavefronts after the gap.

Mastery target

Use Huygens' principle and the wavelength-to-gap comparison to predict, sketch and explain the extent of diffraction.