'The radius of curvature of the phase front.
Examples
>>> from sympy.physics.optics import BeamParameter
>>> p = BeamParameter(530e-9, 1, w=1e-3)
>>> p.radius
1 + 3.55998576005696*pi**2'

@property
def radius(self):

    return (self.z * (1 + ((self.z_r / self.z) ** 2)))
