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When I was reading about the hydrogen spectrum, I came across the statement:

Only Balmer series of hydrogen spectrum can be visible to our eyes.

I really got surprised by that. Why can't other hydrogen spectrum line be visible? They surely are emitted just as well?

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Dhakshin Ram is a new contributor to this site. Take care in asking for clarification, commenting, and answering. Check out our Code of Conduct.
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    $\begingroup$ Because our eyes only see a limited range of energies. Using other instruments many other hydrogen lines are used in astronomy. $\endgroup$
    – Jon Custer
    Commented Jun 27 at 14:03
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    $\begingroup$ See en.wikipedia.org/wiki/Hydrogen_spectral_series - the Lyman series are all in the UV, the Paschen series are all in the IR. Only the Balmer series has lines in the human-visible wavelength region. $\endgroup$
    – Jon Custer
    Commented Jun 27 at 14:08
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    $\begingroup$ You can see the Balmer series lines in my answer here: physics.stackexchange.com/a/768678/313612. As per the wikipedia linked article, the other series beyond the range of typical human vision. They are readily detected by appropriate instrumentation. $\endgroup$
    – Ed V
    Commented Jun 27 at 15:28
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    $\begingroup$ It seems you have been taken by surprise that human eyes are able to detect photons from just a small part of the electromagnetic radiation spectrum. $\endgroup$
    – Poutnik
    Commented yesterday
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    $\begingroup$ I, again, remove some stuff like "present in the universe, propagates in the hydrogen medium" This is chemistry, not astronomy. Hydrogen on Earth certainly shines just as well. $\endgroup$
    – Mithoron
    Commented 18 hours ago

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The reason why we only see the "Balmer Series" is because of its frequency (energy of the photon = Planck's constant × frequency) corresponds to the visible light region in the electromagnetic spectrum.

What is visible light and why (and how) is it visible to us humans?

All electromagnetic radiation is considered light, however only a percentage of electromagnetic radiation, which we refer to as visible light, is visible to humans. Our eyes' cone cells, which are a type of a photoreceptor cell in the retina, function as receivers that are calibrated to detect wavelengths in this particular region of the spectrum. There are wavelengths in other parts of the spectrum that are too intense (too big or small) for our biological senses to handle.(1)

The majority of humans can visually perceive wavelengths between around 400 and 780 nanometers (nm). The limits of the visible spectrum for humans are not well defined; rather, they are spectrums themselves. Colour vision is the outcome of combining signals from three visual pigment types within cones: red, green, and blue, which correlate to cone types L, M, and S (RGB-LMS), respectively. This is also known as trichromatic colour vision. These colours correspond to the wavelengths of peak light absorption intensities of the modified chromophores (a molecule which absorbs light at a particular wavelength and emits colour as a result). Peak absorptions for L cones are found between 555 and 565 nm, M cones between 530 and 537 nm, and S cones between 415 and 430 nm. Therefore, colour vision results from the peak absorption levels of the shifted cones and, in the end, from the brain's interpretation of the makeup of these wavelength absorption points. The complete process is frequently referred to as the "retinoid cycle."(2) (As this is biology related, I won't dive deeper into it on Chem SE.)

Interestingly, ageing processes in the eye cause changes in a person's vision and sensitivity to light during their lifetime. With ageing, the lens's transparency diminishes, particularly for the visible spectrum's short wavelength region (blue light).(3)

References:

(1) https://science.nasa.gov/ems/09_visiblelight/
(2) https://www.ncbi.nlm.nih.gov/books/NBK470227/
(3) https://www.bfs.de/EN/topics/opt/visible-light/introduction/introduction.html

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Because that's how hot the Sun (or more precisely, its photosphere) is. Our eyes are evolved to see things in the peak intensity range of sunlight, which hits specifically on the Balmer series.

A star as massive as our Sun in the Main Sequence is going to have a temperature roughly in the range where most of the black-body radiation is in the 1-3 electron volt range of photon energy. Well, the Lyman series going down to the $n=1$ level involves transitions of 10 eV or more given hydrogen's total ground-state ionization energy of 13.6 eV, and the Paschen series going down to the $n=3$ level is basically all under 1 eV. Like the famous porridge in the children's tale, the Balmer series is just right for the 1-3 eV range where most of radiation, and the solar radiation we are evolved to see, is emitted.

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    $\begingroup$ (+1) You had me at porridge! $\endgroup$
    – Ed V
    Commented Jun 27 at 20:44
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    $\begingroup$ An image might help make this more easily understood. Just a suggestion. :-) (The image I linked is Creative Commons licensed.) $\endgroup$
    – Jason Patterson
    Commented Jun 28 at 0:11
  • $\begingroup$ I do not understand. An image of what? The spectra? The Sun's blackbody radiation? $\endgroup$
    – Oscar Lanzi
    Commented Jun 28 at 0:24
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    $\begingroup$ I linked an image of the transitions. I was suggesting that being able to see the transitions and their relative sizes depicted graphically might make something like a transition to a given value of n make more sense. $\endgroup$
    – Jason Patterson
    Commented Jun 28 at 3:50
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Each series defines the wavelengths you get when an electron drops from a higher orbital. The amount of energy is highly dependent on the orbital it drops to with drops to the first orbital being the most energetic.

Hydrogen_spectral_series

enter image description here By A_hidrogen_szinkepei.jpg: User:Szdoriderivative work: OrangeDog (talk • contribs) - A_hidrogen_szinkepei.jpg, CC BY 2.5, Link

  • Lyman series represents drops to the first orbital and is the most energetic. These photons are all in the ultraviolet range
  • Balmer series represents drops to the second orbital. The most energetic of these are in the visible range, the others are in the infrared.
  • The other series are for the third and further orbitals and thus emit photons with even less energy so they are in the infrared

enter image description here By OrangeDog, CC BY-SA 3.0, Link

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School and college textbooks train our mind to think that answers exists for all questions. Sometimes, there are no known answers to very simple questions. There could be plausible answers but not definite ones. Why do we have two eyes, not compound eyes like a dragon fly? As to why a certain number of hydrogen atoms transitions are visible to humans, the most honest answer would be nobody really knows. Why can't we see ultraviolet like the bees? The hypothesis that human vision evolved to see the visible spectrum of the Sun, which is abundant in hydrogen is a fallacy of causality. Sun emits significant energy in the infrared as well, but humans can feel the effects of effects of infrared but not see it. A hot car on a sunny day reminds the presence of infrared light. The visibility of the Balmer series within the human visible region spectrum is a coincidental overlap rather than a direct causal relationship.

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