What is the wavelength (in nm) of the least energetic spectral line in the visible series of the H atom?
Spectral lines are bright or dark lines appearing in an otherwise continuous spectrum. These can be bright lines in an emmision spectrum (continous black), or dark lines for absorption spectrum (continuous 'color' spectrum). The spectral lines appear at particular wavelengths distinct to every atom. These wavelengths are related to the energy associated with it. Spectral patterns are sometimes referred to as the atomic fingerprint as they can be used to determine the identity of atoms. In fact, these have been used during the discovery of helium (in space!) as well as other elements.
The spectral lines observed for hydrogen are:
410.0 nm (Violet)
434.0 nm (Blue-Violet)
486.1 nm (Blue-Green)
656.2 nm (Red)
Since wavelength is inversely proportional to energy, the least energetic among these is the 656.2nm, red spectral line.