https://www.google.com/url?sa=t&source=web&rct=j&url=https://optics.byu.edu/BYUOpticsBook_2015.pdf&ved=2ahUKEwiyy7jjzb7hAhUFcCsKHVQOA7kQFjAAegQIARAB&usg=AOvVaw1cdBjZ7Td_FiT2_00O_srn