The notion of coherence

The development of the previous expression yields:

We can note that:

The last term represents the term of interference of both fields, it is also called interference modulation. This term is zero if:

• the two waves are perpendicularly polarized, for example if and since then  ; we refer to this as polarization coherence

• both frequencies are very different , since the term in the cosine oscillates so quickly, at a frequency that approaches 1014 Hz and it is not possible to observe with the naked eye or a detector; we refer to this as temporal coherence

• the wave vectors are highly variable in the region of observation   and , since the term in the cosine oscillates very rapidly in space, leading to a null average, and it cannot be observed by the naked eye or by a detector; we refer to this as spatial coherence

These aspects of coherence will be further explained in a later part of the unit.