Diffraction

Besides interference is diffraction the second particular property of the wave nature of light.

Francesco Grimaldi reports in one of his books - published two years after his death - about strange rims of shadows. He observes objects standing in the sun not casting sharp defined shadows. Instead he observes gradual transitions from bright to dark.

"Grimaldi benannte dieses Phänomen des Lichts nach dem lateinischen Wort >diffractio<. Newton, der diesen Vorgang später ebenfalls entdeckte, bezeichnete ihn als >inflection<, was ins Deutsche mit >Beugung< übersetzt werden kann. Diese Bezeichnung deutet bereits darauf hin, daß Lichtstrahlen sich offensichtlich nicht nur gradlinig bewegen, sondern an den Rändern von Objekten aus ihrer Richtung abgelenkt und gebeugt werden."

[Grimaldi named this phenomenon of light after Latin word >diffraction<. Newton, who observed this occurrence too, named it >inflection<. This term already indicates, light rays obviously not move in straight lines , but become diverted and bent at the edges of objects.]

Thereby his observation contradicts that time's assumption of a straight-line propagation of light.

(a) (b) (c)

Fig. Light diffraction

Grimaldi's observation can be explained as followed:

Light impinging onto an object's surface - respectively passing an aperture - is expected to behave as in (a). But it does not. An object's edge - respectively an aperture - works as a point source for a series of elementary waves which, because of diffraction, get bent. The smaller the aperture, the greater light diffraction (compare b and c). Occurring wave fronts superimpose, interfere and consolidate, respectively delete each other. As a result gradual transitions occur. Augustin Fresnel formulates:

"[...] jeder Punkt, der von einer Welle getroffen wird, ist Ausgangspunkt einer kugelförmigen Elementarwelle."

[...every point getting hit by a wave, is the origin of a spherical elementary wave."

He succeeds in proofing light diffraction by his experiment of Fresnel zonen plate.

Fig. Fresnel zone plate

With his discovery Fresnel brings final evidence for validity of wave theory of light.