Coherence

An Interference pattern can not be generated by random light source. Ordered conditions of illumination are needed.

To illustrate this, the example of two stones in entry on interference hitting the surface of the water should assist once again. A constant pattern solely occurs under the condition of synchronization and phasing. For the stones wouldn't hit the water simultaneously or for several stones would hit the water uncontrolled, of course wave fronts would meet and join to a new wave front, but this one wouldn't be constant. This behaviour of waves goes for both, water and light waves.

Viewing figures Partition of white light into different-coloured spectra by prism and Wave lengths of visible light against this backdrop, it becomes obvious that sun light or the light of a candle or an ordinary lamp - like any other light occurring in nature - is just very limited or even not at all suitable to generate constant interference pattern. All these lights consist of several light colours with several different wave lengths. There are superpositions, but they are high complex and chaotic, that no observable interference pattern could be generated with it.

This special property light has to have to generate such an interference pattern is called coherence. Coherent light is ordered and sorted light, which does not propagate in all directions in space, but in just one direction and that consists of waves with just one wave length, i. e. which is monochromatic. As yet is the laser the only source of coherent light.

Wave length

The wave length λ (in nanometer - nm) defines the colour of light. The relation between colour of light and wave length is similar to the relation between intensity of light and amplitude. Therefore different colours of light do have different wave lengths. The human eye can perceive light with a wave length between about 400 nm and 700 nm.

"Lichtquellen, wie die Sonne oder die Glühlampe, senden ein Gemisch aller möglichen Wellenlängen, d. h. Farben aus. Ein derartiges Gemisch von Farben empfinden wir als weißes Licht. Aber selbst, wenn uns Licht einfarbig grün oder rot erscheint, wie etwa das Licht einer Verkehrsampel, enthält es normalerweise noch mehrere, wenn auch nur wenig unterschiedliche Wellenlängen."

[Light sources like the sun or a bulb emit a mixture of all kinds of wave lengths, i. e. colours. Such a mix of colours occurs to the human eye as white light. But even light occurs plain green or red - like light from a traffic light - it normally contains several (although few) different wave lengths.]

Fig. Partition of white light into different-coloured spectra by prism

As the first ever Newton carries out a test as shown in the figure above and publishes his observations in 1704 in his book about optics. About 200 years later physicists like Niels Bohr and Arnold Sommerfeld succeed in assigning different colours of light to different spectral lines with specific wave lengths.

Fig. Wave lengths of visible light

Therefore red light (650 nm, or 6.500 Å - Ångström) is long-wave light, while violet light (430 nm, or 4.300 Å) is short-wave light. Light with a wave length of more than 650 nm is called infra-red. Light with a wave length of less than 430 nm is called ultra-violet. Light of the helium-neon gas laser - which plays an important role later - has got a wave length of 623,8 nm and appears to the human eye as red light.

Every single lightwave illustrated in Fig. Wave lengths of visible light is monochromatic - consists of just one colour of light (respectively has got the same wave length). As coming entries will show, is "[...] monochromatic light [...] of basic importance in holography [...]".