It is given in my book that " If the spectral lines are expresses in terms of Wavenumber, then the visible lines of the Hydrogen spectrum obey the following formula: Wavenumber = 109,677(1/22 - 1/n2) Here, I cannot understand that how can the visible lines be expressed in terms of Wavenumber? Wavenumber is the number of waves per unit length. How is it related to visible lines in the Hydrogen spectrum? Please help. PS- I am a student of 11th class.