Fresnel-Huygens principle
The Huygens-Fresnel principle is a method of analysis applied to wave propagation problems. It is named after the physicists Christiaan Huygens and Augustin-Jean Fresnel, and can be stated as follows:
Every point of an initial wave front can be considered as a source of secondary spherical waves that extend in all directions with the same speed, frequency and wavelength as the wave front from which they come.
This vision of the propagation of waves helps to better understand the phenomena of diffraction, reflection and refraction of waves.
For example, if two rooms are connected by an open door and a sound is produced in a far corner of one of them, a person in the other room will hear the sound as if it originated at the threshold. As regards the second room, the air that vibrates on the threshold is the source of the sound.
The same is true for light passing the edge of an obstacle, but this is not easily observable due to the short wavelength of visible light. Interference of light from areas with varying distances from the moving wavefront explains the observable maxima and minima as diffraction fringes. See, for example, the double slit experiment.
History
In 1678, Huygens proposed that every point hit by a light disturbance becomes a source of a spherical wave. The sum of these secondary waves determines the shape of the wave at any subsequent time. Huygens assumed that secondary waves traveled only "forward" without explaining in his theory why this is the case. He was able to give a qualitative explanation of linear and spherical wave propagation, and to derive the laws of reflection and refraction from this principle, but he could not explain the deviations from rectilinear propagation that occur when light meets with edges, openings and screens, commonly known as diffraction effects.