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Image 1 Caption:
An emission line spectrum
showing the first 4
atomic spectral lines
of the hydrogen
Balmer series
(AKA the Balmer lines).
The Balmer lines
are the atomic spectral lines
of atomic
hydrogen
gas.
The Balmer lines are part of the overall
hydrogen spectral series.
The 4 shown
Balmer lines
are the only Balmer lines in the fiducial
visible band (fiducial range 0.4--0.7 μm).
Image 1 looks like a simulation, NOT a real
photograph.
Features:
- Going in order of decreasing vacuum
wavelength
(i.e., going blueward), the shown
Balmer lines are
Hα
(red),
Hβ
(aqua),
Hγ
(blue),
and
Hδ
(violet).
The Hα
line is often noticeable in astronomical images: it gives that
nebulous pink color in
true-color
images of some gaseous
nebulae particularly
H II regions.
- Note the Balmer lines are
for an
atomic
hydrogen gas
(i.e., a gas of
hydrogen atoms)
and NOT
molecular
hydrogen gas
(i.e., a gas
of hydrogen
molecules
H_2).
Molecular
hydrogen is the normal state
of hydrogen near the
Earth's surface.
However, it is easy to create atomic
hydrogen
gas discharge tube
where heating by an electric current
causes
chemical dissociation
of the molecules.
- How do you get a emission line spectrum?
If you have any relatively dilute hot gas,
it will emit a
emission line spectrum, NOT
a continuous spectrum
like that of white light.
The lines of an emission line spectrum
are nearly all you get when the light
from the gas is
dispersed through a
diffraction grating or
prism.
The background will be
black or nearly so as in Image 1.
- Emission line spectra
are used to identify atoms,
molecules,
and ions
(which could be either atomic
or molecular)
in the technique known
as spectroscopy.
- Image 2 Caption: The emission line spectrum of
hydrogen (H, Z=1)
in the
visible band (fiducial range 0.4--0.7 μm)
(4 lines shown)
and the nearest part of the
near ultraviolet (fiducial range 0.3--0.4 nm)
(2 lines shown,
but they are a bit hard to see).
Image 2 looks like a real
photograph.
- These spectral lines are from
the aforesaid Balmer series.
As aforesaid, there are only 4
Balmer lines in
the fiducial visible band, but the
Balmer series extends
into the fiducial
near ultraviolet and
ideally there is an infinity of
Balmer lines, but
they become increasingly closely spaced as they approach the
Balmer limit λ=3646 Å
(AKA the Balmer
ionization edge)
which is the
ionization
wavelength
of hydrogen from
principal quantum number
n = 2 energy level.
In fact, perturbations always truncate
the Balmer series as it extends
toward the Balmer limit
in a complicated and time-varying way.
- Note that the human eye
does actually see into the
near part of the near ultraviolet
and many Balmer lines
in this band
are detectable by the human eye of most people.
- The first 7
going blueward
(which is the direction of decreasing strength
of absorption lines
in stars) and
the Balmer limit are:
- Hα (6564.5377 Å, red line):
This is the spectral line that
gives the pink glow in many astronomical images.
The glow comes mainly from H II regions which
are ionized hydrogen recombining to
give a
recombination cascade
of many emission lines of
which the strongest is the Hα
which gives the aforesaid pink glow.
- Hβ (4861.3615 Å, aqua line) .
- Hγ (4340.462 Å, blue line).
- Hδ (4101.74 Å, violet line).
- Hε (3970.072 Å, ultraviolet line).
- Hζ (3889.049 Å, ultraviolet line).
- Hη (3835.384 Å, ultraviolet line):
NOT shown in Image 2.
 
 
∞. Balmer limit (3646 Å, ultraviolet line):
NOT shown in Image 2.
Images:
- Credit/Permission:
Merikanto,
before or circa 2006 /
Public domain.
Image link: Wikipedia:
File:Emission spectrum-H.png.
- Credit/Permission: ©
Jan Homann,
2009 /
CC BY-SA 3.0.
Image link: Wikimedia Commons:
File:Visible spectrum of hydrogen.jpg.
Local file: local link: line_spectrum_hydrogen_balmer.html.
File: Spectra file:
line_spectrum_hydrogen_balmer.html.