|Plate of evaporating pellets.|
This is a photograph I took for my Photographic Periodic Table Poster. The sample photograph includes text exactly as it appears in the poster, which you are encouraged to buy a copy of.
I took this photograph because my other black-background iodine photos were not good enough to go with all the other black-background pictures in the poster. I wanted to capture the beautiful purple smoke you get when iodine is gently heated. (The iodine pictured here is of course gone now, having evaporated during photography.)
Interesting fact about iodine vapor: You can't photograph it against a black background. Absolutely cannot be done, period. Now, photographing smoke against a black background is easy: Just shine light in from the side and some of it will be reflected towards the lens, resulting in bright smoke against a dark background. (Some of the prettiest smoke ever is in this story I wrote about making a light bulb.)
But iodine vapor is not smoke. Smoke consists of millions of very fine solid particles, each of which absorbs some light, reflects some, and diffracts some. However, iodine vapor, while it looks for all the world like purple smoke, is in fact a gas consisting of individual molecules of iodine, which can only absorb light, not reflect or diffract it. So any light entering it will either be absorbed, or will continue on in the same direction. Shining light on it from any angle will not result in any going towards the lens, unless the light is shining directly from the back into the lens, which of course is another way of saying that the background is light, not dark.
Colored gases are extremely uncommon. In everyday life, any time you see air that does not appear perfectly clear, it's because there is a particulate smoke of some sort in it. (Fog, for example, is a smoke of water droplets.) You virtually never see colored vapor that isn't smoke, so even though I should theoretically have know it was impossible, I spent about half an hour trying to pour more and more light into the side of my iodine vapor to make it show up against a pitch black background. Only after I had several thousand watt-seconds of studio flash light concentrated on it did I suddenly realize the complete futility of the exercise.
So now you know why iodine is one of very few samples in my poster that are not photographed against a black background.
Source: Ian Brown
Contributor: Ian Brown
Acquired: 15 April, 2006
Text Updated: 29 January, 2009