These highly simplified cartoons show
thin film interference. The first shows a non-reflective coating on glass: a thin layer whose refractive index n is less than that of glass, and whose thickness is λ/4n. (For more details see Non-reflective coatings.) First, consider the case where the difference in optical pathlength (here 2nt =
λ/2) is rather less than the coherence length, which is 2λ in the cartoon. (For light produced from hot objects, the coherence length can be of this order.)
In the cartoon above, we see destructive interference, as expected. The two sine wave segments overlap over most of their length, and they are π out of phase.
In the second cartoon, the difference in optical pathlength is 2nt = 5λ/2, which would produce destructive interference for light from a highly coherent source. However, this is longer than the coherence length of the light in our cartoon, which is again 2λ. Here there is no overlap and so no interference.
In practice, there are usually many photons simultaneously. However, those that have travelled further need not have a correlated phase, because they were emitted by a different event that occurred earlier.