Can a Silver Oxide Coating Start a Fire with Sunlight?

[SAZAR]

I've seen those filters for cameras (etc.) which look black (because they don't pass almost no visible light), but they allow infrared light to go right thorough it.

What is it made of? (i tried to find out on my own using google; I tried to find out what elements absorb what wavelenghts, but I couldn't find it...)

What substance does that (passes whatever and infra-red, BUT NOT visible light)? Can some of substances commonly found in household or wherever do that?

?How to improvise?

 

[brewnog]

The black bits on the end of an exposed and processed photographic film work pretty well in the manner you've described!

A few weeks ago I saw a guy who had replaced the filters in his digital camera (rubbish webcam) with a few pieces of exposed (black) photographic film, he got some really eerie images with it, especially when he used a TV remote control as a torch to illuminate his subjects!

 

[SAZAR]

Wow. OK
I'll try it (...and come to tink of it that I have had it handy all the time...).
I'd never have thought of it.

(...but I was experimenting with black marker and find out that it works very well indeed - I painted a piece of transparent plastic with it - both sides, and multiple times, so you can't see, not even a white wall with sunlight shining directly - right upon it, but camera sees right thorough it (black and white)).

Nevertheless - what molecules have that exact properties (wavelenght absorption of visible light and reflection or passing of the infra-red).

ONE EXPERIMENT: If you paint a magnifying lense with such a coating, would you be able to burn with it when you focuse a sun light? Or, on the other hand - if you would paint a concave miror could you start a fire with it? (as I understand almost 50% of sunlight is in infrared spectrum)

 

[DrMark]

Wow. OK


Nevertheless - what molecules have that exact properties (wavelenght absorption of visible light and reflection or passing of the infra-red).

While this could be accomplished with series of molecules (say porphyrins), it is most easily done with semiconductors. A semiconductor absorbs "all" light with energy above the bandgap. And in fact silver oxide (from the over exposed film referred to above) is a semiconductor (Ebg=1.1 eV or 1127 nm). So "all" visible light will be absorbed by it while the near ir will be transmitted (well most of it imho nir starts ~750 nm).

Wow. OK


ONE EXPERIMENT: If you paint a magnifying lense with such a coating, would you be able to burn with it when you focuse a sun light? Or, on the other hand - if you would paint a concave miror could you start a fire with it? (as I understand almost 50% of sunlight is in infrared spectrum)

The solar spectrum is shown below (AM0). You can see that with a filter such as silver oxide you would loose ~75 % of the total energy available (e.g. you're not going to be toasting bugs with that magnifying glass).