ðə vˈæljuː fɔːɹ ɪz juːnˈiːkli dɪtˈɜːmɪnd fɹʌm fˈɜːst pɹˈɪnsɪpəlz baɪ nˈoʊɾɪŋ ðætðɪ ˈɪnsɪdənt pˈaʊɚɹ ɔnðə dɪskɑːntɪnjˈuːᵻɾi mˈʌst ˈiːkwəl ðə sˈʌm ʌvðə pˈaʊɚɹ ɪnðə ɹᵻflˈɛktᵻd ænd tɹænsmˈɪɾᵻd wˈeɪvz:
The value for  is uniquely determined from first principles by noting that the incident power on the discontinuity must equal the sum of the power in the reflected and transmitted waves: