# Wave-fronts, diffraction & interference

###### Wavefronts

Parallel electromagnetic waves with a common point of origin, the same frequency and phase, and propagating through the same medium, produce an advancing wavefront perpendicular to their direction of travel.

• Lasers that form a pencil of light made of parallel rays produce waves with flat wavefronts.
• An electromagnetic wave with a  flat wavefront is called a plane wave.

Point sources emitting electromagnetic waves in all directions, at same frequency and phase, and propagating through the same medium, produce spherical wavefronts tangental to their origin.

###### Diffraction

Diffraction of electromagnetic radiation refers to various phenomena that occur when a light wave encounters an obstacle or opening.

• Diffraction describes the way light waves bend around the edges of an obstacle into regions that would otherwise be in shadow.
• An object or aperture that causes diffraction is treated as being the location of a secondary source of wave propagation.
• Diffraction causes a propagating wave to produce a distinctive pattern when it subsequently strikes a surface.
• Diffraction produces a circular pattern of concentric bands when a narrow beam of light passes through a small circular aperture.
• In classical physics, the diffraction of electromagnetic waves is described by treating each point in a propagating wavefront as an individual spherical wavelet.
• As each wavelet encounters the edge of an obstacle it bends independently of every other. However, interference between wavelets alters the angle to which they bend and the distance they must travel before striking a surface.
• The explanations that best describe the process of diffraction belong to Wave Theory and are the result of two centuries of study in the field of optics.
• In modern quantum mechanics, diffusion is explained by referring to the wave function and probability distribution of each photon of light when it encounters the corner of an obstacle or the edge of an aperture.
• A wave function is a mathematical description concerning the probable distribution of outcomes of every possible measurement of a photon’s behaviour.