In The Climate Wars  (BBC1) the presenter, Iain Stewart, demonstrated that CO2 absorbs Infrared (IR) energy and so showed how it can trap heat in the Earths atmosphere contributing to the so called 'green house effect' . The apparatus was simple enough; there was a 1m long, 20cm diameter tube which was filled with CO2 from a cylinder. A lighted candle was held at one end of the tube while a thermal imaging IR camera viewed it from the other end. The bright false colour image of the hot candle on the camera screen slowly disappears as CO2 was introduced into the tube showing that the gas absorbs in the IR. I built the apparatus for the program so let me share what I learnt about this experiment.
Due to the vibrations of the atoms in the molecule CO2 has a number of absorption's in the IR, the main bands being at 4.3 µm (2350 cm-1), 7.5 µm (1388 cm-1) and 15 µm (667 cm-1) . The latter band lies very close to the maximum of the Earth's IR black body emission making CO2 a very important greenhouse gas.
The thermal imaging camera we used was sensitive from ca. 1 to 5 µm, quite a large part of the IR spectrum. A lit candle or match produces lots of energy through the IR to the visible. Consequently a candle looks very bright (colourful) on the false colour IR camera image.
In order to be able to seal and look through the tube the ends were covered in plastic cling film. Now plastics absorb strongly in the IR so it's hardly as transparent as it looks to the eye but the film was so thin these simple 'windows' actually worked quite well in practice. The CO2 was flowed in to one end of the tube and vented out the other so that it was well flushed with gas at about atmospheric pressure. In the process the thin film windows bulged a little.
You would think from what I said above that when you view the candle through the tube using the camera, and you introduce CO2 the bright flame would 'disappear' due to the IR absorption. However, when you try this it doesn't work, the candle doesn't disappear!
The reason is that the CO2 absorptions observable by the IR camera are quite weak and are only in a relatively small part of the spectrum. The only way to get the demonstration to work is to have a 'CO2 filter' on the camera. This only lets through IR at around 4 µm, close to one of the CO2 absorption's (which are broadened a bit at atmospheric pressure). The filter blocks out much of the IR energy so that the CO2 absorption is not so swamped anymore and this allows us to now observe our vanishing candle effect.
In the Thomas Crown Affair we talked about how IR cameras usually have an Automatic Gain control (AGC) to make the camera responce as versatile as possible. In this case the AGC will stop the candle from disaperaing as it will re-adjust the gain to try and make it farly constant as the signal drops. So to get this demonstration to work the Camera AGC needs to be turned off.
We used a large diameter tube so that we had the option of seeing the presentors face through the apparatus (see last paragraph below). In the end we used it to observe a candle so we could have had a smaller diameter tube. The 1m path length seemed to work well when the tube was filled with CO2 at atmospheric pressure. If you just wanted to measure an IR absorption and were not worried about getting a recognisable image (of say a candle) then a much smaller length tube could be used if multiple passes were arranged using mirrors.
In the Earths atmosphere the CO2 also absorbs in bands but of course there is a much greater amount of gas and also other greenhouse gases present. We also think that global feedback systems in the ocean for example, increase the amount of CO2 available as things heat up multiplying the amount of warming greenhouse gases still further.
An alternative USA series on climate change  recently did the same experiment using a military state-of-the-art IR camera and CO2 filter (unfortunately not available for us to use in the UK). It was so sensitive that they used the heat from the presenters face rather than a candle to demonstrate the effect.
To see a YouTube clip of the experiment please go to: CO2 experiment
 The Climate Wars, BBC1 Sept. 2008, see: web site
 A real glass greenhouse actually works by preventing convection not by IR absorption! See the Wikipedia section on 'Greenhouse' effect :
greenhouse effect on Wikipedia
 Greenhouse Gas Absorption spectrum in the Earths atmosphere
 Global Warming: what's up with the weather, Nova, DVD. ISBN 978 1 59375 729 8
How teachers can use these articles in a lesson
Why Hollywood Science
Open University Hollywood Science web site
Call for clips - do you have a film clip that needs investigating?
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