# Snell's law in MetaPost

Snell's law tells us how a monochromatic light travels through a boundary between two different materials: $$n_1\sin\alpha=n_2\sin\beta$$

The $$n_1,n_2$$ values are constants determined by the respective materials.

This makes for fun MetaPost drawings. The code below draws a prism with a ray of light being refracted according to Snell's law.

Sometimes the angle and materials puts $$|\sin(\alpha)n_1/n_2|>1$$, in that case all the light gets reflected inside the first material. If we mess with the requirements we can make it really difficult for the light to escape from the prism:

Open question: If the prism is a convex regular polygon with $n$ sides, what is the smallest $$n$$ where $$\alpha$$ exists such that the light never escapes the prism? Seems impossible for $$n=3$$ but for a circle very easy, I guess the answer would be a function of $$n_1,n_2$$.

outputformat:="svg";
outputtemplate:="%j_%c.%{outputformat}";
hppp:= 0.2;
vppp:= 0.2;

vardef asin   primary x = angle((1+-+x,x)) enddef ;
vardef angleat expr t of p = angle (direction t of p) enddef;

u:= 2cm;

path p[],bb,kb;
bb:=unitsquare scaled 4u shifted (-2u,-1u);
pair A,B,C,h;
A=-B=(1,0)*u;
C=(0,2)*u;
path tri;
tri:=A--B--C--cycle;

numeric a[],b[],n[];
n1=1;
n2=1.4;

%Parameters
% h:= .4[B,C]; %Incidence location
% a1:=40; %alpha angle (0-90)

h:= .4[B,C]; %Incidence location
a1:=8; %alpha angle (0-90)

% h:= .2[B,C]; %Incidence location
% a1:=50; %alpha angle (0-90)

beginfig(1);
draw tri;
draw bb;

p := h--(dir ((angleat .5 of (B--C)) + 90 + a1)*3u shifted h);
draw p1 cutafter bb;
p2=p1;
boolean outside,leave;
outside:=true;
leave:=false;
forever:
h:= p2 intersectionpoint tri;
t:= ypart(p2 intersectiontimes tri);
ray:=angleat 1 of p2;
normal:=(angleat t of tri)-90;
if outside:normal:=normal+180;fi
%    draw h--(dir(normal)*1u shifted h) dashed evenly;
if ray*normal>0:
a1:=ray-normal;
elseif ray>normal:
a1:=-360+abs(ray)+abs(normal);
else:
a1:=360-abs(ray)-abs(normal);
fi
show "alpha:";
show a1;
refi:= if outside :sind(a1)*n1/n2;else:sind(a1)*n2/n1;fi
if (refi > 1) or (refi < 0) : %physics says -1, 0 is pretty;
show "reflection";
p2 := h--(((dir(normal-a1))*3u) shifted h);
else:
show "refreaction";
p2 := h--(dir(normal + 180 + asin(refi))*3u shifted h);
if outside=false:leave:=true;draw p2 cutafter bb;fi
fi
p2 := (subpath(epsilon, 1) of p2);
draw (p2 cutafter tri) cutafter bb;
p2 := reverse(p2);
outside:=false;
exitif(leave);
endfor;

endfig;
end