8/6/2019 Photochromic Lenses Handout

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Aida Habibelahi

Photochromic Lenses-Applications of Chemistry

Photochromic Lenses:

Photochromic lenses are lenses that darken when exposed to ultraviolet radiation. When the

radiation is removed by walking indoors, the lenses will gradually return totheir clear state

Photochromic lenses are made up of molecules of substances such as silverchloride or silver halide. The molecules are transparent to visible light in

the absence of UV light, but when exposed to UV rays, the molecules

undergo a chemical process that causes them to change shape. The new

molecular structure absorbs portions of the visible light, causing the lensesto darken.

Because photochromic compounds fade back to their original state by a

thermal process, the higher the temperature, the less dark photochromiclenses will be. This thermal effect is called "temperature dependency" and

prevents these devices from achieving true sunglass darkness in very hotweather. Photochromic lenses will get very dark in cold weather conditions.

Photochromic Lenses and Redox Reactions:

In photochromic lenses, silver chloride (AgCl) and copper (I) chloride (CuCl) crystals are added

during the manufacturing of the glass. These crystals become uniformly embedded in the glass.

One characteristic of silver chloride is its susceptibility to oxidation and reduction by light.

Cl-

-----------> Cl + e-

oxidation

Ag+ + e- ---------> Ag

reduction

The chloride ions are oxidized to produce chlorine atoms and an electron. The electron is then

transferred to silver ions to produce silver atoms. These atoms cluster together and block the

transmittance of light, causing the lenses to darken. This process is reversible which allows thelenses to become transparent again. The presence of copper (I) chloride reverses the darkening

process. When the lenses are removed from the light, the following reactions occur:

Cl + Cu+ ------> Cu+2+ Cl-

The chlorine atoms formed by the exposure to light are reduced by the copper ions, preventingtheir escape as gaseous atoms. The copper (+1) ion is oxidized to produce copper (+2) ions,

which then reacts with the silver atoms:

Cu+2+ Ag ------> Cu+1+ Ag+

http://en.wikipedia.org/wiki/Lens_(optics)http://en.wikipedia.org/wiki/Ultraviolethttp://en.wikipedia.org/wiki/Sunglasshttp://en.wikipedia.org/wiki/Lens_(optics)http://en.wikipedia.org/wiki/Ultraviolethttp://en.wikipedia.org/wiki/Sunglass

8/6/2019 Photochromic Lenses Handout

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Aida Habibelahi

The effect of these reactions is that the lenses become transparent again because the silver and

chloride atoms are converted to their original oxidized and reduced states.

Photochromic lenses and Le Chateliers Principle

AgCl + energy Ag+ + Cl-

(clear) (dark)

When you go outside, there is more intense light (energy) than there is inside, therefore the

equilibrium shifts to the right and the glass darkens.

When you go back inside, there is less intense light (energy) than there is outside, therefore the

equilibrium shifts to the left and the glass lightens (becomes clear)

Photochromic lenses and rate of reaction

The amount of light, the type of radiation, the temperature of the lens, and the thicknessof the lens are all factors that affect the rate of reaction at which photochromic lenses

change from their clear state to a dark colour, and reverse back to a clear state.

The amount of light-

Darkening occurs much more quickly with a clear blue sky than with overcast conditions

or when inside a car. This is because more light is available which causes the rate ofreaction to increase.

The type of radiation-The activating waveband is within the UVA and blue region of the visible spectrum. When

there is a large amount of this waveband (eg. At high altitudes), the rate of reaction increases

and the photochromic lens darkens faster.

The temperature of the lens-

Photochromic lens performance increases as the temperature of the lens decreases. When it iscold, the rate of reaction is increased and when it is warm, the rate is decreased.

The thickness of the lens-

When the photochromic lens is thicker, there is a greater density and concentration of thesilver halide molecules. This allows the reaction to occur faster which causes the lenses to

darken faster than it would if the lens was thinner.