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Class 11 Chemistry (India)
Course: Class 11 Chemistry (India) > Unit 2
Lesson 5: Bohr's model of hydrogen atom- Bohr's model of hydrogen
- Absorption/emission lines
- Absorption and emission
- Emission spectrum of hydrogen
- Bohr model radii (derivation using physics)
- Bohr model radii
- Bohr model energy levels (derivation using physics)
- Bohr model energy levels
- Atomic Energy Levels
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Absorption/emission lines
Broken rainbows
In 1801 William Wollaston observed a rainbow in close detail and noticed tiny dark lines in the visible spectrum. Here is an experimental setup for the prismatic observation of the solar spectrum by Wollaston:
In 1817 Fraunhofer took an even closer look at the Sun’s spectrum (IR, UV and visible) by expanding the spectrum onto a large wall. As a result, he found thousands of slices that were missing.
These are known as absorption lines or ‘Fraunhofer lines’. But why do they occur?
Colorful gasses
A major advance was made in 1859 by Gustav Kirchhoff and Robert Bunsen that explained why the dark lines occur. When certain chemicals were heated in Bunsen's burner, characteristic bright lines appeared.
In some cases these were at exactly the same points in the spectrum as Fraunhofer's dark lines. For example, these are the spectrum lines visible when we heat hydrogen gas. This is known as the Balmer series :
Light passing through cooler atmospheric hydrogen will show absorption lines in the exact same position. For example imagine sunlight passing through a tube of hydrogen gas and then split using a prism:
Emission lines refer to the fact that glowing hot gas emits lines of light, whereas absorption lines refer to the tendency of cool atmospheric gas to absorb the same lines of light. When light passes through gas in the atmosphere some of the light at particular wavelengths is scattered resulting in darker bands. These lines came to be known as ‘spectral lines’ and were cataloged by heating common elements until they produced light and measuring the wavelengths emitted.
Below is an interactive illustration. Click on different gasses to see how they absorb different wavelengths of light:
This provides a fingerprint that confirmed the presence of chemical elements at a distance. Astronomers use this fingerprint (which spans the entire spectrum) to determine the composition of distant stars (which are essentially balls of burning gas) as well as the atmosphere of stars and planets (that absorb light passing through their atmospheres).
Want to join the conversation?
The interactive app seems to be very useful for the next challenge. Can someone convert it into an interactive app in KA (using ProcessingJS) and embed it into the next challenge?(17 votes)
The program was written using the KA CS environment. You can see it here: https://www.khanacademy.org/computer-programming/interact-absorption-lines/2226438064(4 votes)
In the interactive illustration--I entered Nitrogen and Oxygen both together. Since that was the composition in the model "atmosphere" I expected most of the colors (except blue) to be absorbed. Sure... there is LOT of blue left... but there is also lots of green, violet, yellow. In previous videos- it says the sky is blue because that wavelength is scattered the most/reflected to us- so we see blue. Seems to me, not much of the spectrum is really absorbed and we should see lots of colors all the time (not just at sunset). Thanks for your thoughts!(7 votes)
This is a very good question. They say in the video 'Visible light (1672)' that it is JUST ENOUGH nitrogen and oxygen to scatter other light and keep the blue, (which there is probably some hydrogen in there, too). There is probably some carbon and sulfur during the day, too. During a sunset when there is mainly the red and yellow light coming through, it looks like there is mainly sodium, oxygen, and hydrogen to scatter the blue light.(2 votes)
I asked it in Polish but there are nobody :( . I don't speak English very well, but I have been learning English for long time( 7 years ?!), so I’m going to try.
Can I see emission spectrum of gas or absorption spectrum (at home or at school), how?
I haven’t got prism, but maybe at school is diffraction grating – at home “CD” works [good word?] like professional diffraction grating.
Can I burnt gas in Bunsen burner? hmm… or use gas-discharge lamp…
Can I burnt salts metals in alcohol (like “flame test”) for example: NaCl and dispersion yellow light?
“…imagine sunlight passing through a tube of hydrogen gas and then split using a prism…” [from: https://en.khanacademy.org/partner-content/nasa/measuringuniverse/spectroscopy/a/absorptionemission-lines] Can I do this ?
Thanks, you finished read text where I broke English ;)(5 votes)- Your English is fine. When the sun, or any other light, reflects off of a CD, the rainbow light pattern that you see is an emission spectrum, so you can see an emission spectrum at home simply by holding a CD to some light source. I have also seen people burn salts, such as table salt or KCl in a bunsen burner. Salts with different metals will emit different colors, and those different colors are the emission spectrum of that element. If you have the materials to do this at home then you should be able to see the spectrum.
I hope this helps!(5 votes)
If the absorption/emission lines refer to the colour of the flame when the element is burned, why are there so many different colours on the spectrum if the reaction only results in one colour?(2 votes)
I think both the absorption and emission lines are showing which colors are being absorbed. You are supposed to look at the dark areas of the absorption spectra and those dark areas indicate that the color which would be there is being absorbed. This means that if there is a big dark band where blue would be, then the opposite color to blue on the color wheel is being transmitted. (This would be orange.) The element hydrogen turns orange when being burned and this color is transmitted to us. Therefore, all the other colors would be absorbed. If you look at the lines for hydrogen; blue, purple, and red are being absorbed. However, there are MORE dark lines in the blue region. So if blue is being absorbed, the opposite color would be transmitted and this color is orange. Remember, always look at the color area on the rainbow that is blacked out the most. This is the color that will be the opposite of the flame color on the color wheel. All the other colors shown are just part of the natural light being shown down on the element. Actually, if you just burned hydrogen and looked at its spectra, you would get the Emission Spectra and not the Absorption Spectra, and this Emission Spectra would only show the bunch of blue lines, one purple line, and one red line. And these are being absorbed (with emphasis on blue). All the colors of the Absorption Spectra do make it kind of confusing. I think this is when white light is used that you get an Absorption Spectra.(5 votes)
- But why do chemicals absorb certain wavelengths of light? Why do they absorb any light?(3 votes)
Light is energy. Energy is transferred between matter. Some things absorb less light. The light that certain chemicals might absorb is just a property of that chemical, like color or hardness or smell.(1 vote)
I clicked on one (or more) and there is no blue.
Why is it like that?(3 votes)- Not every element absorbs blue light. For instance, some things reflect blue light. Example: blueberries(1 vote)
- Could emission and absorption lines be visible simultaneously?(3 votes)
why the sky is blue in colour ?(1 vote)- Are there also gas or liquid absorption spectra for the infrared spectrum just like the visible spectrum shown? Which liquid or gas is most infrared adsorptive across the infrared spectrum?(2 votes)
- How do you determine the element which light bypasses if you only have the spectral lines?(1 vote)
Each element has different spectral lines. Kind of like a fingerprint, the lines tell us which elements are present.(2 votes)
