The Fresnel–Arago laws are three laws which summarise some of the more important properties of
interference between light of different states of
polarization.
Augustin-Jean Fresnel and
François Arago, both discovered the laws, which bear their name.
Two parallel coherent linearly polarized waves will interfere in the same way as
natural light.
The two constituent orthogonal linearly polarized states of natural light cannot interfere to form a readily observable interference pattern, even if rotated into alignment (because they are incoherent).
Formulation and discussion
Consider the interference of two waves given by the form
where the boldface indicates that the relevant quantity is a
vector. The
intensity of light goes as the electric field
absolute square (in fact, , where the angled brackets denote a time average), and so we just add the fields before squaring them. Extensive algebra [2] yields an interference term in the intensity of the resultant wave, namely:
Now it can be seen that if is perpendicular to (as in the case of the first Fresnel–Arago law), and there is no interference. On the other hand, if is parallel to (as in the case of the second Fresnel–Arago law), the interference term produces a variation in the light intensity corresponding to . Finally, if natural light is decomposed into orthogonal linear polarizations (as in the third Fresnel–Arago law), these states are incoherent, meaning that the phase difference will be fluctuating so quickly and randomly that after time-averaging we have , so again and there is no interference (even if is rotated so that it is parallel to ).