Friday, 31 January 2014

How the Greenhouse Effect works

Here is a rather pleasing demonstration of how carbon dioxide can absorb infrared radiation. It was originally set up by the awesome Jonathan Hare and you can read more about the design here. The BBC wanted to replicate the demo for Stargazing Live that transmitted in early January 2014, and asked me to do the honours.

Much to my relief, we managed to get the demo up and running about two hours before the show was due to go live. It was working beautifully and showed the effect really clearly. So much so, that we even managed to substitute a face for the candle. 

And then they dropped it from the show, half an hour before live transmission.

It's what happens to demos on TV. They make easily excisable chunks of content. When I had been told after the rehearsal they were running 7 minutes over - I could see the writing was on the wall for this one. Rather than just dejectedly pack it away, I got Rob King from the Met Office to film me talking through the demo:


...

So, what's going on? 

Carbon dioxide gas absorbs infrared radiation at a number of specific wavelengths. The main absorption peaks are at 4.26 microns and a broader peak at 15.5 microns. The infra red camera was filtered so that it only saw infrared radiation with a wavelength of 4.26 microns (+/- 0.05 microns). Infrared radiation of this wavelength travels from the candle, through the tube, to the camera and we can see the hot flame on the screen. When carbon dioxide is flooded into the tube, the gas absorbs the infrared and the flame disappears. 

This is the mechanism behind the Greenhouse Effect that can be seen on both Venus and Earth. Light from the Sun, across a range of wavelengths, hits the planet surface, is absorbed and warms the surface. The warm surface then re-emits this heat as infrared radiation at a specific wavelength of about 15 microns. This corresponds to the absorption peak for carbon dioxide that we didn't use in the demo, but the effect is the same. Heat that was on its way back out to space is captured and held in our atmosphere. The atmosphere warms and on Venus you get run away Global Warming, while on Earth we get comparatively minor anthropogenic Climate Change. 

For those seeking to replicate this you will clearly need a big Perspex tube sealed at the ends with cheap cling film. Read through Jonathan's notes for construction suggestions. In my case the carbon dioxide came from a home brew system called Midget Widget, which is a great light weight gas supply. The camera was a crucial sticking point for us. Most infrared cameras can only see radiation from about 7 to 15 microns, which puts both carbon dioxide absorption peaks outside of their range. Cameras that see near-infrared (i.e. 4.26 microns) used to be common, but not so anymore. The big manufacturer of IR cameras, Flir do make them, but only for research purposes. We finally tracked one down at the Met Office and they kindly lent it to us. The filter was a bit easier and came from Northern Optical Coatings who manufacture filters for Flir. Once you have the camera and filter sorted, the demo is pretty simple to set up and worked first time for me.

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Science TV Presenter, live show performer, writer, strange prop builder and all round Science Bloke. All opinions expressed are mine alone. Not the BBC's, just mine.