class_docstr: Represents an optical medium for which only the refractive index is known.
  Useful for simple ray optics.
  
  This class should never be instantiated directly.
  Instead it should be instantiated indirectly by instantiating Medium with
  only n specified.
  
  Examples
  ========
  >>> from sympy.physics.optics import Medium
  >>> m = Medium('m', n=2)
  >>> m
  MediumN(Str('m'), 2)
code: def n(self):
      return self.args[1]
  
imports: from sympy.physics.units import second, meter, kilogram, ampere
  from sympy.core.basic import Basic
  from sympy.core.symbol import Str
  from sympy.core.sympify import _sympify
  from sympy.functions.elementary.miscellaneous import sqrt
  from sympy.physics.units import speed_of_light, u0, e0
label_desc: Use image node_id 2 for calling the MediumN obj's underlying member method code with example usage: obj.n() and returns: self