code: def geometric_conj_ab(a, b):
      """
      Conjugation relation for geometrical beams under paraxial conditions.
  
      Explanation
      ===========
  
      Takes the distances to the optical element and returns the needed
      focal distance.
  
      See Also
      ========
  
      geometric_conj_af, geometric_conj_bf
  
      Examples
      ========
  
      >>> from sympy.physics.optics import geometric_conj_ab
      >>> from sympy import symbols
      >>> a, b = symbols('a b')
      >>> geometric_conj_ab(a, b)
      a*b/(a + b)
      """
      a, b = map(sympify, (a, b))
      if a.is_infinite or b.is_infinite:
          return a if b.is_infinite else b
      else:
          return a * b / (a + b)
  
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 3 for calling a global function with example usage: geometric_conj_ab(a, b) and returns: unknown, unknown