http://www.seab.gov.sg/aLevel/20102011Syllabus/9646_2011.pdf
SECTION lV     WAVES  Superposition
[s](d)        explain the meaning of the term diffraction.

(e)        show an understanding of experiments which demonstrate diffraction including the diffraction of water waves in a ripple tank with both a wide gap and a narrow gap. [/s]

(f)         show an understanding of the terms interference and coherence.

(g)        show  an  understanding  of  experiments  which  demonstrate  two-source  interference  using water, light and microwaves.


may have to finer customized some more the meaning for coherence.

i argue that the best pedagogy method is in this case learning by making
the best way to demonstrate understanding of coherence is to get students to use Ejs to change the codes inside.
hint:
v1 = f1*lambda1
v2 = f2*lambda2
should do it.
when v1 =! v2 & f1=!f1 & lambda1=! lambda2 it will result in phi1=! phi2 in general except for those time of coincidence!

The less pedagogy effective method but more time efficient method is to make it happen in the sim by teacher.

the trade offs of deep pedagogy learning vs efficient time.