An orange emission line is observed at 656.3 nm in the hydrogen emission spectrum. determine the initial energy level of the electron.

The energy of a wave is calculated by E = hf where f is the frequency and h is the Planck's constant (6.626 ×10^-34 J/s).
The formula may be written as E= hc/λ where c is the speed of light 3×10^8 m/s and λ is the wavelength.
Thus energy = (6.626 × 10^-34)×(3 ×10^8)/ (656.3 nm)
                     =  3.0267 × 10^ -19 J
 Therefore, the initial energy level of an electron is 3.0267 × 10^-19 joules.