deBroglie's formula allows us to switch momentum for wavelength:

                      h
         lambda  =  -----
                      p


                      h           
         lambda  =  -----         
                      px         



  In a classical single-slit diffraction pattern, the center
  of the screen is bright, and dark spots appear when an angle
  theta leads to destructive interference:


        d sin(theta)  =   lambda


                             h
                      =    -----
                            px


  Solving for theta, we get


                              (  lambda  )           (    h     )
              theta   =  asin ( -------- )   =  asin ( -------- )
                              (    d     )           (  d * px  )