[hamdani yusuf (OP)]

It was.
I looked at a test tube full of hydrogen.

Can you tell us more about your experience? Do you have any pictures?
What's the characteristic of the scattered light? Is it bright enough to be seen with naked eyes?
How is the distribution of the scattered light? Does it produce reflected light?
If the laser is polarized vertically, does the gas scatter the light upward?
I'm sorry if I ask too much. Your help will be greatly appreciated.

[Bored chemist]

Can you tell us more about your experience?

It looked like a test tube full of air.

[hamdani yusuf (OP)]

Can you tell us more about your experience?

It looked like a test tube full of air.

It looks like I have to do the experiment myself.

[Bored chemist]

Can you tell us more about your experience?

It looked like a test tube full of air.

It looks like I have to do the experiment myself.

What do you expect to see?
Hydrogen looks like air.

[hamdani yusuf (OP)]

What do you expect to see?
Hydrogen looks like air.

I expect to see scattered light if the frequency is in Balmer series, and no scattered light when it's not.
If the scattered light is too dim we can just increase the intensity of the laser beam.

[hamdani yusuf (OP)]

No visible light interacts with hydrogen except via scattering.

If this is true,  then the absorption spectrum of hydrogen is a misnomer. We should call it scattering spectrum instead.

[Bored chemist]

No visible light interacts with hydrogen except via scattering.

If this is true,  then the absorption spectrum of hydrogen is a misnomer. We should call it scattering spectrum instead.

Not really.
The absorption spectrum of hydrogen gas near room temperature all the way from the microwave up to the vacuum ultraviolet  is a flat line at zero.
Hydrogen gas does not absorb any of that EM radiation.

If you have a spectrum that shows any absorptions then it is wrong, or mislabeled somehow.

[Bored chemist]

It's important to remember that hydrogen gas in a tube is composed of molecules, rather than atoms.

Does it apply for both emission and absorption spectra?

If the laser has a very narrow bandwidth, eventually all electrons in level 2 will go up to level 3. This will make the gas unable to absorb the laser anymore. Is there an experiment demonstrating this hypothesis?

Yes, and I have done that sort of experiment when i was a student (with an NMR transition, rather than a visible one, but the principle is the same),

More generally
https://en.wikipedia.org/wiki/Spectral_hole_burning
and
https://en.wikipedia.org/wiki/Saturable_absorption

[chiralSPO]

I think it is also important to note that the Balmer series (and Lyman, and Paschen etc.) are for atomic hydrogen, not molecular hydrogen (H vs H₂). H is not stable at standard temperatures and pressures, but is stable at temperatures where electronically excited H atoms are thermally accessible.

[hamdani yusuf (OP)]

How hot does it take for H2 gas to turn into atomic H? I guess it's significantly lower than melting point of glass.

[Bored chemist]

I guess it's significantly lower than melting point of glass.

Very roughly 10,000 degrees.
Hot enough to boil glass (and anything else).

[Bored chemist]

I learned that the gas could turn into plasma when exposed to microwave, or high AC voltage from Tesla coil. But they are relatively low frequency radiation compared to what causes photoelectric effect on metals.

You can run an arc with DC.
The frequency is not relevant.

[hamdani yusuf (OP)]

I guess it's significantly lower than melting point of glass.

Very roughly 10,000 degrees.
Hot enough to boil glass (and anything else).

So,  the absorption spectrum can't be observed in a desktop laboratory equipments? How did they produce those data?

[hamdani yusuf (OP)]

One more thing, does the hot gas emit observable thermal radiation? Is there any resemblance to the black body radiation?

[chiralSPO]

Emission spectra can be produced in a bench-top (or hand-held) vapor discharge lamp. Low pressure H₂ can be broken down by a high voltage: https://www.flinnsci.com/hydrogen-gas-spectrum-tube/ap1334/

As an undergraduate student in a general chemistry laboratory class, I used such devices (for many different elements) as well as a diffraction grating to observe their emission spectra.

I have observed the absorption spectrum of H by analysis of the light from the sun (dark line spectrum, or "Fraunhofer lines")

[hamdani yusuf (OP)]

Emission spectra can be produced in a bench-top (or hand-held) vapor discharge lamp. Low pressure H₂ can be broken down by a high voltage: https://www.flinnsci.com/hydrogen-gas-spectrum-tube/ap1334/

As an undergraduate student in a general chemistry laboratory class, I used such devices (for many different elements) as well as a diffraction grating to observe their emission spectra.

I have observed the absorption spectrum of H by analysis of the light from the sun (dark line spectrum, or "Fraunhofer lines")

I've seen a diagram in a physics article showing that hot hydrogen gas produces emission spectra while cool hydrogen produces absorption spectra. How far from the truth can it be?

[chiralSPO]

I've seen a diagram in a physics article showing that hot hydrogen gas produces emission spectra while cool hydrogen produces absorption spectra. How far from the truth can it be?

Cold hydrogen will not emit, only hot. Both hot and cold will absorb.

[evan_au]

does the hot gas emit observable thermal radiation? Is there any resemblance to the black body radiation?

Yes, the Sun is composed of hot gas (primarily line spectrum), and even hotter plasma (primarily black body radiation).
- In a hot gas, the electrons are in defined shells around the nucleus, with defined energy levels. This produces a line spectrum (absorption and emission lines with specific photon energies)
- In a plasma, electrons are not bound to any particular nucleus, but can take any energy approaching or leaving the vicinity of a nucleus (or another electron). This produces a continuous spectrum (black body radiation, with photons of all energy levels).
See: https://en.wikipedia.org/wiki/Sunlight#Composition_and_power

[hamdani yusuf (OP)]

I've seen a diagram in a physics article showing that hot hydrogen gas produces emission spectra while cool hydrogen produces absorption spectra. How far from the truth can it be?

Cold hydrogen will not emit, only hot. Both hot and cold will absorb.

So, this diagram is misleading then?

https://casswww.ucsd.edu/archive/public/tutorial/Stars.html
University of California, San Diego
Center for Astrophysics & Space Sciences
We may consider three principal types of spectra which appear when the light from an object is broken up into its component wavelengths or "dispersed":

a continuous spectrum or continuum; the emission of a thermal spectrum is one type of continuum.
an absorption spectrum or sometimes an absorption-line spectrum.
an emission spectrum or emission-line spectrum.

I found many like this on line, such as.

and

[hamdani yusuf (OP)]

I guess it's significantly lower than melting point of glass.

Very roughly 10,000 degrees.
Hot enough to boil glass (and anything else).

Hydrogen discharge tubes are available on line. They produce Balmer spectrum without causing the glass to boil.