Arago spot

The Poisson spot is a diffraction phenomenon of optics and describes the bright spot in the center of a diffraction pattern with diffraction of light by a circular opaque object.

All the points of a circle have the same distance to each point on the vertical axis through the center of the circle. For vertical illumination of the object all elementary waves therefore interfere constructively on the whole axis.

According to Huygens' principle, all overlap from one edge outgoing spherical elementary waves, so spread this also in the geometric shadow behind opaque objects. By this diffraction of light resulting in the superposition of all to be considered elementary waves determined interference pattern that can be visualized on a suitable screen.

History

Many scientists, as well as Isaac Newton, then refused a wave- theoretical analysis of light, such as by Christiaan Huygens and were unchanged from the fact that it is light is a Teilchenphänomen whose trajectories could be described purely mechanical. It was the then scientists do not yet know that light can be fully described only when the quantum mechanical wave -particle duality is the basis.

In order to approach the theoretical causes of the many unexplained optical observations, in 1818 was awarded by the French Academy of Sciences a competition. The 30- year-old engineer Augustin -Jean Fresnel participated in this competition with a novel work on the wave theory of light. The jury, who was also Siméon Denis Poisson, was directed by François Arago.

Poisson studied the submitted work of Fresnel, but was convinced after reading of the particle nature of light. With a simple thought experiment he tried the validity of Fresnel's theory to refute: After his reflections based on the theory would have to be evidence Light behind a round, lighted object in the center of the shadow, because the light from the entire circular edge of the obstacle according to the theory as wave would interfere constructively there. However, since this was not according to experience and obviously the case, the ideas of Fresnel would be absurd. Poisson This raised consciously against the already described by Christiaan Huygens wave nature of light.

Arago decided, however, to perform this experiment under very precise conditions and could actually prove the existence of such a light spot, by attaching a metal disc with a diameter of two millimeters with wax on a glass plate and illuminated .. This was a strong and convincing argument for the given predicted wave nature of light, and to the chagrin of Poissons eventually won Fresnel competition.

The phenomenon has been described already in 1715 by Joseph -Nicolas Delisle and 1723 by Giacomo Filippo Maraldi whose works were initially but remained largely unnoticed.

In the English literature, the term Poisson 's spot is used as a synonym Arago 's Spot.