Photochromic Lenses: Mechanism

Photochromic lenses have millions of molecules of substances such as silver chloride or silver halide embedded in them. The molecules are transparent to visible light in the absence of UV light, which is normal for artificial lighting. But when exposed to UV rays, as in direct sunlight, the molecules undergo a chemical process that causes them to change shape. The new molecular structure absorbs portions of the visible light, causing the lenses to darken. The number of molecules that change shape varies with the intensity of the UV rays.

http://science.howstuffworks.com/question412.htm


Can anyone help me with the mechanism of this reaction? Silver halides aren't molecules, and I'm really curious how they "change shape" when they absorb uv photons.

I don’t know the answer to this question but what I do know might be a clue:
UV light consists of photons of higher energy than photons of visible light -so high that they tend to much more often ionise molecules by knocking off electrons off molecules and if the electron that is knocked off is that of a covalent bond then that generally means that covalent bond is broken and this results in a free radical that then chemically reacts (often but not necessarily by forming a new covalent bond) to virtually whatever other molecule it comes into contact with (except very inert molecules such as SiO2 ) In this way UV light can cause chemical reactions. Not sure though exactly how this relates to UV causing molecules of Silver halides to specifically “changing their shape” although it may involve ionisation of the molecules.

Originally posted by Andrew Hamilton
I don’t know the answer to this question but what I do know might be a clue:
UV light consists of photons of higher energy than photons of visible light -so high that they tend to much more often ionise molecules by knocking off electrons off molecules and if the electron that is knocked off is that of a covalent bond then that generally means that ...[text shortened]... ides to specifically “changing their shape” although it may involve ionisation of the molecules.

I don't think there are any chemical reactions involved here.

I think that maybe they change position with one another. I'm just guessing here, but you can imagine all these molecules aligned horizontally. When a photon of enough energy clashes with it, it rearranges, aligning in another direction.

This could account for the change of color.

The only information I can find indicates that Silver Halide crystals are used for the lenses.
When a photon hits a valence electron on the Silver Halide, it jumps the band gap into the conduction band. The electron travels until it is trapped by a shallow electron trap. The electron will reduce an interstitial silver ion to form atomic silver.

I have no idea how that reverses in the absense of UV light.

Originally posted by mlprior
The only information I can find indicates that Silver Halide crystals are used for the lenses.
When a photon hits a valence electron on the Silver Halide, it jumps the band gap into the conduction band. The electron travels until it is trapped by a shallow electron trap. The electron will reduce an interstitial silver ion to form atomic silver.

I have no idea how that reverses in the absense of UV light.

As I was reading about photographic paper, I read that the silver atoms are not stable in this situation, but will spontaneously decompose unless you get a certain concentration of them (I think). The photographic paper has clusters of silver atoms which, because they are clustered, will not decompose.

I think that this might be the answer, but someone really smart is going to have to let you know for sure.

The electrons are in orbits at the lowest, stable, energy state. When a UV photon comes along, it's like getting a swift kick and jumping up. That absorbed kick was invisible to the human eye because that range of energy doesn't serve any survival purpose. However, the return to the resting (lower) energy state (sitting back down and relaxing again) involves emitting the absorbed energy at a visible wavelength. That's because it's important for us to be able to see the object (ions).

Originally posted by coquette
I think that this might be the answer, but someone really smart is going to have to let you know for sure.

The electrons are in orbits at the lowest, stable, energy state. When a UV photon comes along, it's like getting a swift kick and jumping up. That absorbed kick was invisible to the human eye because that range of energy doesn't serve any survival p ...[text shortened]... isible wavelength. That's because it's important for us to be able to see the object (ions).

I think you just described fluorescence.