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Objects appear in different colours because they absorb some colours (wavelengths) and reflect or transmit other colours. The colours we see are the wavelengths that are reflected or transmitted.

As far as I understand, when an atom absorbs a photon, one of its electrons gets excited (= unstable). So it drops back to the ground state, emitting that energy in the form of photons of a specific colour.

This means that we should find objects to be in certain colours. But, for example, hydrogen gas is colourless.

How is this possible? Shouldn't Hydrogen gas have a specific colour related to its emission spectra?

I was reading an article on this topic today (https://www.khanacademy.org/science/chemistry/electronic-structure-of-atoms/bohr-model-hydrogen/a/spectroscopy-interaction-of-light-and-matter), and this came into my mind.

Can anyone please help in clearing my understanding?

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    $\begingroup$ It is also biological, there is color perception hyperphysics.phy-astr.gsu.edu/hbase/vision/colper.html ,See also my answer here physics.stackexchange.com/questions/605951/… $\endgroup$
    – anna v
    Commented May 23, 2023 at 18:43
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    $\begingroup$ The vast vast majority of most objects whose colour you are perceiving aren't emitting a single wavelength of that same colour. It's emitting a mix and your brain is doing an arbitrary interpretation. After all, people with red-green blindness might see your red as green and your green as red. Totally arbitrary. $\endgroup$
    – DKNguyen
    Commented May 24, 2023 at 1:26
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    $\begingroup$ Thanks for posting your question! Hopefully, the combination of posted answers has been helpful! $\endgroup$
    – Ed V
    Commented Jun 19, 2023 at 16:56

4 Answers 4

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At room temperature, hydrogen gas exists as diatomic molecules that do not significantly absorb light in the visible spectrum, i.e., 400 nm to 700 nm. So hydrogen gas at room temperature is colorless. In a hydrogen gas discharge tube, that is not energized by an appropriate high voltage source, the hydrogen gas is colorless, as shown in my photograph below:

Hydrogen gas discharge tube

Application of high voltage, via an appropriate high voltage power supply, results in a mixture of excited hydrogen atoms and molecules. Both exist in the discharge and both emit light as they continually cycle around their respective excitation and de-excitation pathways.

This figure shows my photographs of the energized hydrogen discharge tube.

Energized hydrogen discharge tube

On the left is the raw output. To my eyes, it is quite noticeably red hued. On the right, a filter has been used to attenuate light at wavelengths longer than 600 nm. The light transmitted through the filter is, to my eyes, cyan hued.

Others may see the colors differently, but no matter: we have spectrometers, spectrographs, and the like. So, collecting light from the energized hydrogen discharge tube, and dispersing it using one of my homemade echelle spectrographs results in the following two dimensional spectrum (an echellogram):

Annotated hydrogen echellogram

I have annotated the echellogram to show grating orders, the Balmer line wavelengths, in angstroms, and a little about the acquisition of the echellogram. N.B. 1 angstrom = 0.1 nm.

As noted above, the echellogram shows the four Balmer series lines in the visible spectrum. Each is present twice, in consecutive echelle grating orders, as a consequence of how echelle spectrographs may operate. The Balmer lines are from excited hydrogen atoms emitting light as they de-excite. The other light is from other excited species, e.g., excited hydrogen molecules.

Note particularly the Fulcher alpha bands in the longer wavelength region of the echellogram. These have been know for many years and are useful for various purposes, e.g., in plasma diagnostics. A spectrum in conventional format, i.e., intensity versus wavelength, is shown below:

Fulcher alpha bands

So it is a bit more complex than just those nice Balmer lines. The neat thing about spectroscopy is that the more you look, the more you see.

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  • $\begingroup$ The cheap plasma globe spectral lines illustrate how spectra may be relatively simple if there are no molecules involved: astronomy.stackexchange.com/a/49116/45954. So the hydrogen discharge tube is actually a more complicated system than the plasma globe with neon and xenon gases in it. $\endgroup$
    – Ed V
    Commented Jun 19, 2023 at 2:22
  • $\begingroup$ See also purple, the glorious mixture of colors that is not 385 nm: physics.stackexchange.com/a/754027/313612. See also the “line of purples” article in wikipedia and links therein. $\endgroup$
    – Ed V
    Commented Jun 19, 2023 at 12:03
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    $\begingroup$ I hope someone can appreciate the efforts that went behind that one echellogram! $\endgroup$
    – ACR
    Commented Jun 19, 2023 at 15:02
  • $\begingroup$ Many thanks for accepting my answer! I enjoy doing answers like this! $\endgroup$
    – Ed V
    Commented Sep 9, 2023 at 1:56
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H absorbs very specific wavelengths, leaving almost all wavelengths undisturbed. Also the excited atoms then decay, producing the same wavelengths that were just absorbed.

Those wavelengths are the same that are produced in a gas discharge lamp, which is not at all colorless. The reason is that the lamp produces lots of light at those wavelengths. The excess is very noticeable.

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Atomic hydrogen gas does glow with a particular colour, as long as it is at sufficient temperature to excite some of its atoms into excited levels. Here is an example:

First an RGB image taken with broadband filters. The reddish colours in this image are due to the Balmer alpha emission of hydrogen gas at 656 nm.

Horsehead no H alpha

To show this, here is a similar image but this time the red is taken from a narrow-band filter centred on the Balmer alpha wavelength, clearly showing that the "redness" originates from that particular wavelength. [Images taken from Utah desert remote observatories].

Horsehead H alpha

Of course molecular (H$_2$) gas does not exhibit these colours because it is a molecule and transitions between molecular ro-vibrational levels are at infrared wavelengths.

As for why more everyday items are coloured - it has little to do with the wavelengths of atomic transitions. It is due to the solid state electron band structure that occurs when you pack atoms together in a solid material and which leads to a variety of reflectance and absorption properties.

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  • $\begingroup$ Then the emission spectra do not really affect the colour of things around us? I mean the absorption of some wavelengths and reflection of some other wavelength thing? $\endgroup$ Commented May 23, 2023 at 17:14
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    $\begingroup$ @Golden_Hawk Most of the things arounds us are not at the temperatures of thousands of Kelvin necessary to excite atomic transitions. $\endgroup$
    – ProfRob
    Commented May 23, 2023 at 18:35
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    $\begingroup$ @Golden_Hawk a few things are, we call them "fluorescent". Got an UV flashlight? If not, get yourself one - it's a neat little toy to see what things glow when you shine UV on them. (don't shine it at anyone's eyeballs, including yours, including in a mirror. Don't shine it around cheap motel rooms if you don't want to feel disgusted) $\endgroup$ Commented May 23, 2023 at 23:41
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As far as I understand, when an atom absorbs a photon, one of its electrons gets excited (and unstable). So it drops back to ground state, emitting that energy in the form of photons of a specific colour.

Other answers did not respond to this bit, but it’s important to realize that it is false in a lot of cases.

The electron does get excited and it is true that a system containing excited species is in a metastable state. The system wants to return to stable. Emitting a photon at the same wavelength as the absorbed photon is one way to do that. Note that this would make the material colorless, but also opaque. You would not be able to see the difference with diffuse reflection under typical viewing conditions.

However, the excess energy may also be dissipated in other ways; a common way is to dissipate it as heat.

For many materials that’s the whole story: absorption simply heats the material. Depending on which wavelengths are absorbed, you perceive a certain color.

It is reasonably common that some energy is dissipated as heat, and the remainder is emitted as a photon. The you observe a combination of the things described above; note that the dissipation may take a long time, such as in the case of fluorescent materials.

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