\magnification=\magstep1

$$
n_1 \sin( \theta_1 ) \quad = \quad n_2 \sin ( \theta_2 )
$$

$$
{\rm extra\ distance\ } x \quad = \quad d \sin (\theta) 
$$

$$
d \sin (\theta)  \quad = \quad m \lambda \qquad , \quad m = 1, 2, 3, ...
$$


\bye
