\magnification=\magstep2

$$
\eqalign{
{\rm light\ ray\ 'B'\ } & {\rm travels\ extra\ distance\ \ } 2L \quad 
                  {\rm (for\ small\ angles)  } \cr
              & {\rm has\ wavelength\ in\ coating\ \ }
                     \lambda_{\rm coat} = ({{n_1}\over{n_2}}) 
                     \thinspace \lambda_{\rm air} \cr
              & {\rm has\ phase\ shift\ of\ \ } {{1}\over{2}} \lambda  \quad
                    {\rm at\ reflection\ with\ silicon\ if\ \ } n_2 < n_3 \cr
}
$$

$$
  { { 2L } \over { \lambda_{\rm air}({{n_1}\over{n_2}}) } }  \quad = \quad 
                      {N + {{1}\over{2}} \qquad N = 0, 1, 2, 3, ... } 
$$

$$
L \quad = \quad {{1}\over{4}} \thinspace \lambda_{\rm air} 
            \thinspace ({{n_1}\over{n_2}}) (N - 1) 
             \qquad N = 0, 1, 2, 3, ...
$$
\bye