Amplitude

The amplitude Ê is a measure for the intensity (brightness) of light. For from a graphical point of view the amplitude describes the height of a wave crest, respectively the depth of a wave valley, measured from an imaginary horizontal line running right through a sinusoidal curve, light with a large amplitude is bright light, while darker light has got smaller amplitude.

"Die Amplitude einer Schreibtischlampe ist größer als die einer Streichholzflamme, die Amplitude des Sonnenlichts wiederum größer als diese beiden.

Mit der Entfernung ändert sich die Amplitude des Lichts. Nahe bei seiner Quelle ist das Lichts stärker und wird immer schwächer, je weiter es sich davon entfernt. Das erklärt sich durch die Tatsache, daß sich Licht auf seiner Reise ausbreitet. Die Energie einer ganzen Wellenfront bleibt zwar gleich, aber die Intensität wird geringer, da die Front eine größere Fläche abdeckt. Sind wir z. B. doppelt so weit von der Lichtquelle entfernt, als vorher, beträgt die Intensität nur noch ein Viertel."

[The amplitude of a desk lamp ist larger than the amplitude of a match's flame. The amplitude of sunlight is larger than both of them.

The amplitude of light changes with the distance. Close to its source light is stronger and gets weaker the farther it removes. This circumstance is explained by the fact that light propagates on its journey. The energy of all of a wave front remains the same, but the intensity gets smaller, for the front covers a larger area. For light departs twice the distance from its source, its intensity amounts just a quarter.]

Referred to the graphical illustration of the sinusoidal curve this means: "[...] if one wave has twice the amplitude of another, the first light has four times the intensity of the second."

"Das ist das Gesetz vom inversen Abstandsquadrat, welches besagt, daß die Lichtintensität umgekehrt proportional zum Quadrat der Entfernung abnimmt. Bei dreifacher Distanz beträgt die Intensität also nur noch ein Neuntel."

[This is the law of inverse square of the distance, which says, that light intensity declines inverse proportional to the square of the distance.]

Fig. Law of inverse proportional light intensity

The following figure would like to illustrate the above described:

Fig. Intensities of different light waves

The waves in (a) and (b) have got the same wave length, but the light in (a) is more intense (brighter) than in (b) - (a) has got a larger amplitude than (b). The ratio between these to waves is about 2:1. Therefore light (a) is four times (2²) brighter than light (b). (c) has got he same intensity as (b), but a larger wave length. (d) again has got the same intensity as (b), but a shorter wave length.

The sinusoidal is therefore a graphical illustration of intensity variation of the energy of light, that travels as a particle photon wave-like through space and time.

Fig. Light photon passes two cycles

It passes a whole cycle, starting at »off« (zero point), passing »on« (wave crest), back to »off« (zero point), passing »on« (wave valley), and back to »off« (zero point).

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.