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 to
their clear state
Photochromic lenses are made up of molecules of substances such as silver
chloride 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 lenses
to darken.
Because photochromic compounds fade back to their original state by a
thermal process, the higher the temperature, the less dark photochromic
lenses will be. This thermal effect is called "temperature dependency" and
prevents these devices from achieving true sunglass darkness in very hot
weather. 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 + eoxidation
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 the
lenses 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+ ClThe chlorine atoms formed by the exposure to light are reduced by the copper ions, preventing
their escape as gaseous atoms. The copper (+1) ion is oxidized to produce copper (+2) ions,
which then reacts with the silver atoms:
Aida Habibelahi
Cu+2+ Ag ------> Cu+1+ Ag+
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 thickness
of 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 lightDarkening 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 of reaction to increase.
The type of radiationThe 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 lensPhotochromic lens performance increases as the temperature of the lens decreases. When it
is cold, the rate of reaction is increased and when it is warm, the rate is decreased.
The thickness of the lensWhen the photochromic lens is thicker, there is a greater density and concentration of the
silver halide molecules. This allows the reaction to occur faster which causes the lenses to
darken faster than it would if the lens was thinner.