class_docstr: Representation for a gaussian ray in the Ray Transfer Matrix formalism.
  
  Parameters
  ==========
  
  wavelen : the wavelength,
  z : the distance to waist, and
  w : the waist, or
  z_r : the rayleigh range.
  n : the refractive index of medium.
  
  Examples
  ========
  
  >>> from sympy.physics.optics import BeamParameter
  >>> p = BeamParameter(530e-9, 1, w=1e-3)
  >>> p.q
  1 + 1.88679245283019*I*pi
  
  >>> p.q.n()
  1.0 + 5.92753330865999*I
  >>> p.w_0.n()
  0.00100000000000000
  >>> p.z_r.n()
  5.92753330865999
  
  >>> from sympy.physics.optics import FreeSpace
  >>> fs = FreeSpace(10)
  >>> p1 = fs*p
  >>> p.w.n()
  0.00101413072159615
  >>> p1.w.n()
  0.00210803120913829
  
  See Also
  ========
  
  RayTransferMatrix
  
  References
  ==========
  
  .. [1] https://en.wikipedia.org/wiki/Complex_beam_parameter
  .. [2] https://en.wikipedia.org/wiki/Gaussian_beam
code: def z_r(self):
      return self.args[2]
  
imports: from sympy.core.expr import Expr
  from sympy.core.numbers import I, pi
  from sympy.core.sympify import sympify
  from sympy.functions.elementary.complexes import im, re
  from sympy.functions.elementary.miscellaneous import sqrt
  from sympy.functions.elementary.trigonometric import atan2
  from sympy.matrices.dense import Matrix, MutableDenseMatrix
  from sympy.polys.rationaltools import together
  from sympy.utilities.misc import filldedent
label_desc: Use image node_id 4 for calling the BeamParameter obj's underlying member method code with example usage: obj.z_r() and returns: self