According to the hydrogen emission spectrum definition when there is no external Energy influence hydrogen is in its ground state ( electron in the fist shell or level). To ionise the hydrogen, we must supply Energy so that electron can move from the first level to infinity. The measurement of the distance between the first and infinity level is called Ionisation Energy. By determining the frequency, we can determine the Energy required for the first level to infinity (point of Ionisation). From this result, we can calculate the Ionisation Energy.