\magnification=\magstep2


$$
y(x, t) = 
            A \cos(k x - \omega t)
$$

$$
{{\partial^2 y} \over {\partial x^2} } =  \quad
      - k^2 \quad A \cos(k x - \omega t)
$$

$$
{{\partial^2 y} \over {\partial t^2} } =  \quad
      - \omega^2 \quad A \cos(k x - \omega t)
$$

$$
 k^2 \quad A \cos(k x - \omega t)  =  
      \quad ( {{k^2}\over{\omega^2}} ) \quad  \omega^2 \quad A \cos(k x - \omega t)
$$

$$
{{k} \over {\omega}} = {{ 2 \pi / \lambda } \over { 2 \pi f }}  = 
          { {1} \over { f \lambda }}
$$

$$
{{\partial^2 y} \over {\partial x^2} } \quad = \quad
           {{1}\over{V^2}} 
        \quad {{\partial^2 y} \over {\partial t^2} }
$$

$$
{{\partial^2 E} \over {\partial x^2} } \quad = \quad
           {{1}\over{c^2}} 
        \quad {{\partial^2 E} \over {\partial t^2} }
$$

$$
{  {\nabla^2 E}  } \quad = \quad
           {{1}\over{c^2}} 
        \quad {{\partial^2 E} \over {\partial t^2} }
$$



\bye
