Light waves pass through slits in a barrier 2a wavelengths apart (left).
Wave interaction can be seen (middle). Interference pattern recorded at distance d is shown (right).
Option-click animation button for best results.

a=slider([0,6])

d=slider([10,100])

z=branch(if(2*[if(sin(2*pi*[x-n]),abs(y)<7)],x<0),if(sin([2*pi*[sqrt(x^2+[y-a]^2)-n]])+sin([2*pi*[sqrt(x^2+[y+a]^2)-n]]),0<x<d),if([1-[sum(e^(-[y-((2*k+1)/4*sqrt(d^2/(a^2-[(2*k+1)/4]^2)+1))]^2)+e^(-[y+(2*k+1)/4*sqrt(d^2/(a^2-[(2*k+1)/4]^2)+1)]^2),k=0,2*a-1)]],x>d))

abs(x)<0.25,abs(abs(y)-a)>0.25

Graph of the formula

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