The coherence of a light field can be domonstrated by the phenomenon of interference. In the classical experiment of Thomas Young in 1801 the coherence was induced by arranging that the light diffracted by a fine slit should illuminate two parallel slits which then acted as two coherent sources. In the more modern version of the experiment a laser beam illuminates the two slits; the transverse coherence across the laser beam creates two coherent sources. The temporal coherence between the two sources can be displayed by superimposing the field E(t) from one slit with the field from the other at a different time E(t + τ). This is achieved by observing the intensity at points that are at different distances, s 1 and s2, from the two slits so that one is superimposing the light fields from two different times t 2 − t 1 = τ = (s 1 − s 2)/c; see Fig. 18. The field at the observation point P at time t′ originates from the slit s 1 at time t 1 = t′ − s 1/c = t; and originates from the slit s 2 at a later time t 2 = t′ − s 2/ c = t + τ.