Plotting the Cornu spiral
MATL, 29 26 25 bytes
Thanks to @Adriaan for 3 bytes off!
Q:qG/q1e3YytP+1e6j*ZeYsXG

Here's an example with input 365 366... because today is MATL's first birthday! (and 2016 is a leap year; thanks to @MadPhysicist for the correction).
Or try it in MATL online! (experimental compiler; refresh the page if it doesn't work).

Explanation
Q:q    % Input N implicitly. Push range [0 1 ... N] (row vector)
G/     % Divide by N, element-wise
q      % Subtract 1. This gives NP projected onto the x axis for each mirror element
1e3    % Push 1000. This is NP projected onto the y axis
Yy     % Hypotenuse function: computes distance NP
tP     % Duplicate, reverse. By symmetry, this is the distance SN
+      % Add. This is distance SNP for each mirror element (row vector)
1e6j   % Push 1e6*1i
*      % Multiply
Ze     % Exponential
Ys     % Cumulative sum
XG     % Plot in the complex plane