Science Buddies: "Ask an Expert"

My grandson is doing the experiment with absorption and heat radiation from different colors of paper. I’m hoping you can explain to us why visible (and above?) photons from the sun, which are absorbed by the electron orbitals in the paper, aren’t radiated back out at the same frequency when the electrons jump back down to their original orbitals. Why/how does the energy that came in at higher frequencies get radiated out at infrared wavelengths?

Also can you confirm that the reflectivity of the different colored papers is only relevant to visible frequencies – so we can assume that the sun’s infrareds and frequencies above violet are absorbed equally by each color of paper?

Hi wayneb,

The background information for the experiment might help answer your question about why the emissions are in the infrared frequency range. See if you can find explanations of the “Terms and Concepts”, and then try to answer the “Questions” near the end. Please post again if you have additional questions while searching.

https://www.sciencebuddies.org/science- ... background

The assumption that the absorption of infrared and ultraviolet light is independent of paper color seems reasonable, since neither frequency is in the visible range. I would expect, instead, that their absorption would depend on the chemical composition of the paper, including any dyes. The assumption might be incorrect, however, if the chemicals used to dye each color paper react differently to infrared and ultraviolet light.

Hi,

This site suggests that the absorption of UV light at least can be more complicated than just the color of the object.

http://m.wisegeek.com/what-is-a-uv-absorber.htm

Hi wayneb,

dfcoll gave you some excellent advice. I did, however, want to add a few additional details.

All objects that have a temperature emit electromagnetic radiation: the Sun, the Earth, you, me, your computer--anything that has a temperature. However, the wavelength distribution of that energy depends on the temperature of the emitting object. Objects with higher temperatures emit most of their energy at shorter wavelengths. Things with cooler temperatures emit most of their energy at longer wavelengths.

The "Black Body Emission Curves" shown on the following webpage show this quite nicely:

http://www.acs.org/content/acs/en/clima ... lance.html

The x-axis of this figure is the wavelength of emitted energy. The y-axis is basically the amount of energy being emitted. Visible wavelengths are indicated by the "rainbow" colors. The distribution of photons emitted by the Sun (yellow curve) "peaks" in the visible wavelength range. The distribution of photons emitted by the Earth (red curve) "peaks" in the infrared wavelength range (centered at ~10 microns) because the temperature of the Earth is much cooler than the temperature of the Sun.

The papers in this science project are essentially the same temperature as the Earth. Therefore, they radiate most of their energy in the infrared. If the papers were (somehow) the same temperature as the Sun, then the papers would emit most of their energy at visible wavelengths.

I hope this helps. Post back if you have more questions or want some additional explanation.

Thanks Terik,

I understand all that. I think we're assuming that the sun's infrared frequencies absorbed by the paper are not relevant, and are being absorbed at the same rate by each color of paper; and the same for the ultraviolets.

I think what we're investigating is visible frequencies being absorbed differently by different colors of paper.

But aren't the visible frequencies only being absorbed by electron orbitals in the dye molecules? I didn't think visible frequencies get absorbed directly as molecular heat. Then when a dye molecule electron jumps back down to its original lower-energy orbital, don't we expect it to radiate the same frequency as it originally absorbed - in which case the dyes would be radiating visible frequencies, rather than infrared.

So my question is how is it that high frequency energy being absorbed into electron orbitals gets converted to molecular motion heat, so we can measure it being radiated as infrared?

Thanks again,

W

Thanks Dfcoll,

I did read the background information before posting my inquiry. It says:

The electrons tend to return to the energy level they were at before they absorbed the photons, called the ground state. In the process, they re-emit the energy as infrared photons.

My understanding though is that in general energy radiated by electrons jumping back to ground state is radiated in the specific amount of the difference in orbital energys, which will always be exactly the same as the energy absorbed to jump up from ground in the first place. But the statement above is implying that infrared photons emitted as the dye electrons jump back to ground have less energy than the visible-frequency photons that were originally absorbed to bump the electron up to the higher orbital.

Energy is getting lost in this scenario, unless somehow a dye electron is simultaneously emitting multiple infrared photons which together exactly match the energy of the visible photon originally absorbed. But I don't think that's the answer to why the wavelengths emitted are longer than the wavelengths absorbed. I'm missing something.

W

Hi wayneb,

This page might help answer your question:
https://en.m.wikipedia.org/wiki/Fluorescence
(Take a look at the section on photochemistry.)

Also take a look at this one:
https://en.m.wikipedia.org/wiki/Franck-Condon_principle

There are multiple pathways available to get back down to the ground state. Certain vibrational energy states can be accessed during the relaxation, depending on the system.

Hope that helps. Please post again if the information raises additional questions.