H-alpha is a red spectral line, at a wavelength of 6563 Å (656.3 nanometers), emitted by a hydrogen atom when its electron falls from the third lowest energy level (n = 3) to the second lowest energy level (n = 2). The same line appears in absorption when electrons are raised from n = 2 to n = 3.

 

Many solar features, such as prominences, show up best in H-alpha, so that, for observation of the Sun, filters are often used that allow only light close to the H-alpha wavelength to pass.

 

H-alpha is the first member of the Balmer series.