Huygens (principle of)
The Huygens principle concerns the propagation of light in the form of transmitted wavelets and re issued successively. It's the astronomer and Dutch mathematician Christian Huygens (1629-1695), that must be the first theorization of the wave front. It was inspired by the properties of mechanical waves, which he pulled the principle of the waves 'envelopes', which since door its name.
By observing the fact that a ripple produced on the surface of the water after the fall of a rock is spread by close in close action, he understood that any point of this reaches surface by the wave should its direct neighbouring points action and movement not to the action of the primary source (place of the fall of the stone). In this theory, any point reached by the wave then acts as a secondary source which, in turn, radiates a Wavelet in direction of the neighbourhood. As a result, the status of the vibration of the surface of the water at a given time allows to predict its future position: it is not necessarily need to know the characteristics of the initial pulse.
Huygens transposed these findings to light and applied that a light source emitted successive impulses, giving rise to a disturbance located on a surface called "wave front", which was spreading in a specific environment ("ether"). At the time of Huygens, the notion of electromagnetic field was not known, and light waves should necessarily have a 'support' for this spread. Huygens designed the process of spread of light Wavelet and mechanical vibrations in an elastic medium.
If a wave front has a certain position at a given moment t, his later position is determined as the envelope of wavelet issued at this time t by points distributed secondary sources its original position. In a homogeneous medium, the waves retain their plan or spherical character.
Light propagation and interference
The principle of Huygens has been completed and mathematically formulated by Fresnel at the beginning of the 19th century, including the concept of interference; "this concept explained that there was no wave re issued"backward"when" light vibration "contiguous sources. Indeed, in a dense homogeneous medium, there is little or no light scattered sideways or backward. Earth's atmosphere contains millions of molecules in a cube which stops it would be equal to a visible light wavelength (ex: 500 nm). This is related to the size of the molecules, the nanometer, while the visible light wave is of the order of a micron.
Huygens principle and law of Snell-Desmaps
In examining the consequences of the Huygens principle, it is important to realize that the emitted wave fronts consecutively are separated by equal time intervals. This property allows to prove the law of refraction of Snell (known as Snell-Desmaps in France). Refraction occurs when the wave front meets a different environment and that its rate of spread is reduced.
Snell's law to calculate the trace of a refracted Ray, from the angle of the incident angle with the normal to the surface and the value of the indices of refraction of each community.
Principle of Huygens and Wavefront
Thus, the very general principle of Huygens was be interpreted in the context of undulatory theory of light (electromagnetic waves). Suppose a light monochromatic (S) in a homogeneous medium isotropic. It emits light waves, which propagate along rays in all directions: in each point of the space close to the source earned by the light wave, the State of the electromagnetic field oscillates (at the frequency of the source), between a minimum value and a maximum value. At some point, all points located at a similar distance from the source are in the same State (they are "in phase").
When the source is placed in an isotropic medium, the envelope of the points located at the same distance from the source shape a spherical space (and circular in plan). The Wavefront is spherical, and because the waves move away from the source, it is divergent. As you move away from the source, the radius of curvature increases (the same portion of wave front is more flat). If one is far enough away from the source, an instrument of observation may collect a portion of the wave surface whose curvature can be considered to be zero (flat).
The wave front is the envelope that connects these points "in phase". flat when the rays spread parallel, it is spherical and expansion as part of a wave front differ. She is also spherical but tends to be collapse to a central point in Wavefront case converge.