def optical_depth(extinc_coef,path_length):
  """
  Optical depth
  
  Given the extinction coefficient and the path length in the same
  system of units, returns the optical depth.
  
  @param extinc_coef : extinction coefficient in 1/cm
  @type  extinc_coef : float
  
  @param path_length : depth of gas cloud in cm
  @type  path_length : float
  
  @return: float
  """
  return extinc_coef * path_length