(hydrogen line for electrons settling from n = 3 to n = 2)
H-alpha (Ha, Hα, or H-α)
is the first line in the Balmer series
of hydrogenspectral lines.
It has a wavelength of 656.3 nm (in air; 656.5 nm in a vacuum)
and is red.
It is within the visible-lightatmospheric window, thus observable from
the ground and is the Fraunhofer C line.
It is bright in the sky but does not stand out visually
because it is so red as to be outside the most sensitive part
of the eye's light sense.
An H-alpha filter (or hydrogen-alpha filter)
is a narrow-bandwidth filter centered on the H-alpha wavelength.
They have many uses, but are a basic tool in the study of the Sun.
Deuterium's H-alpha wavelength is about 0.2 nm shorter.
With spectroscopy of sufficient spectral resolution,
it produces a discernible feature on the H-alpha spectral line,
enabling an estimation of the ratio between 1H and deuterium.