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.

Wave nature of light

Christian Huygens, Augustin Fresnel and Thomas Young are considered as the three main protagonists, who construct a theory of light between 17th and early 19th century, which is looking for help of explanation on the water. Using the example of waves on the surface of the water the nature of light is demonstrated.

"This theory does not depend on knowing what light really is, but on the assumption that it consists of transverse waves, as do water waves, and that the basic wave phenomena occur in much the same way in both cases. This simple theory of the behaviour of light is completely satisfactory for understanding all of the aspects of holography [...]"

This very simple and easy to understand analogy should serve as a paragon to explain for holography fundamental characteristics of light.

When a stone hits the surface of the water, it causes oscillations.

Fig. Propagation of water waves outgoing from a point source

The caused waves propagate in all directions - outgoing from a point source. They move up and down, vertical to the direction of propagation.

"For this reason, such waves are said to be transverse. (In contrast, sound waves in air consist of regions of compression and rarefication along the direction of travel and are called longitude waves). That water waves are transverse is shown convincingly by considering a bobber on the surface, rising and falling vertically as the traveling waves pass it."

Fig. Propagation of water waves - bird's-eye view and cross section

The left part of the figure shows circular propagation of waves from a bird's-eye view. The dotted lines indicate wave valleys, the consistent lines indicate wave crests. To pay a correct terminology back in its own coin, these circles should be named wave fronts. The right part of the figure shows a wave front in cross section as sinusoidal curve. Using the example of sinusoid some terms should be explained, which are going to be used afterwards.

The distance between two wave valleys, respectively two wave crests, describes the wavelength (in figure distance A-A', and B-B'). For a stone now hits the surface of the water exactly at the same position in exact equal intervals, a pattern of a sinusoid curve occurs (respectively a pattern of waves on the surface of the water), which remains steady. For the sinusoid is divided right in its middle by an imaginary horizontal line (dotted line in figure), the distance between the line an maximum of a wave crest, or the maximum of a wave valley, is called amplitude (not illustrated in figure). The passage from zero point to the top of a wave crest, back to zero point, to the low of a wave valley, back again to zero point describes one complete oscillation - one cycle. The frequency measures the amount of times a cycle passes a certain point in one second. It is specified in hertz (hz).

The just explained terms are valid for any kind of sinusoidal curves - anyway if sound, water or light. One significant difference between a water wave and a light wave should not remain unmentioned, to avoid any potential misunderstanding in coming entries about interference and the recording of wave fronts. While a water wave moves up and down, which means at a certain position wave crest and wave valley alternate, a light wave is a standing wave. The wave pattern - position of wave crest and wave valley - does not change.