Huygens'principle

Principle

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The Huygens principle is a method of analysis applied to problems of wave propagation. This holds for macroscopic phenomena (optical devices such as lenses, prisms etc. very much larger than the wavelength). It recognizes that each point of an advancing wave front is in fact the center of a source of a new train of waves and that the advancing wave as a whole may be regarded as the sum of all the secondary waves arising from points in the medium already traversed.

Fig. 1 Huygens' principle applied to refraction when a light beam goes from medium with a high speed of propagation C1 and consequently low refractive index to medium C2 with a low speed and high refractive index.

The Huygens' principle also holds for (near-)microscopic phenomena (optical devices such as apertures and slits of the order of a wavelength). It simply states that a large hole can be approximated by a collection of many small holes so each is practically a point source (whose contribution is easy to calculate). A point source generates waves that travel spherically in all directions). Similarly a relatively wide slit is composed of many narrow ones (subslits), and adding the waves produced by each produces the diffraction pattern (se Light: diffraction). For example, if two rooms are connected by an open doorway and a sound is produced in a remote corner of one of them, a person in the other room will hear the sound as if it originated at the doorway. As far as the second room is concerned, the vibrating air in the doorway is the source of the sound. The same is true of light passing the edge of an obstacle, but this is not as easily observed because of the short wavelength of visible light.

Fig. 2 Huygens' principle applied to diffraction.

The above views of wave propagation helps better understand a variety of wave phenomena, such as refraction and diffraction. The former is visualized in Fig. 1 and the latter in Fig. 2.