How Do Some Prescription Glasses Change
Into Sunglasses Under the Sun?

By: Nadia Rana



     Certain eyeglass companies have invested in developing pairs of lenses that combine prescription glasses and sunglasses into one convenient frame. Also known as photochromic glass, these special light sensitive lenses invented in the '60s are formed using a silver chloride compound as a coating in the lens. The compound ranges from a concentration of .01 to .001 percent. In addition to the silver chloride, copper (I) chloride is also applied to the lenses. The key to the tinting of the glasses is in the sunlight. As sunlight passes through the lenses, UV rays affect the silver chloride by separating the two ions. The UV light, plus the Cl- ion combine to make Cl° + e-. This free e- combines with Ag+ to form Ag°. The chargeless silver and chlorine atoms build up during the reaction to the UV rays, and develop a blue-greyish layer along the surface of the lens.
     You might be wondering how the eyeglass wearer is able to see through this surface after it is coated with silver and chlorine atoms. Well, when the lens is first coated with the ions, the process is carried out in such a manner as to allow them to cool into tiny silver chloride crystals. Since the crystals are so small, the longer wavelengths of visible light prevent them from being seen by the human eye. However, the shorter UV light rays are able to interact with them in order to produce the desired tint.
     Now you may be thinking--how do you undo the tint? Well, it can be explained very easily (though I'm sure that to inventor Stanley D. Stookey it wasn't so simple at first). When the lenses are not in heavy sunlight (say, indoors in a classroom), very little UV light rays are passing through. Because the UV is no longer part of the chemical reaction, the silver and chlorine free atoms can be manipulated. The copper (I) chloride interacts with the Cl to form copper Cu²+ and Cl-. Then, the Cu²+ combines with Ag to form Cu+ and Ag+. The Cl- and Ag+ rejoin once again to form the original silver chloride compound, and the copper returns to its original copper (I) chloride form as well.


If photochromic glasses fascinate you, and you'd just love more
info on them, here are a few sites that might be of interest...


  * DETAILED INFO ABOUT THE CHEMICAL REACTIONS INVOLVED IN THE PROCESS

  * INFO ON THE INVENTOR OF PHOTOCHROMIC LENSES     (pdf file)

  * PURCHASE A PAIR OF PHOTOCHROMIC LENSES FOR YOURSELF!


Here are some nifty experiments you can try in the privacy of
your own home if you're interested in photochromic lenses:*)

*Does the temperature of the lens affect the rate of the reaction?

     To test if temperature is a contributing factor to the reaction rate, place 1 photochromic lens in the refridgerator, and one near a heater or toaster. After 10 minutes, expose both lenses to direct sunlight. Note the time it takes for the cold lens and the hot lens to change color.
*If the lenses are under water, but still exposed to the UV rays of the sun, will the photochromic lenses still change color (considering that the UV rays would have to penetrate the water before the lens)? If so, will the results be less dramatic?

     Place 1 lens in a clear tank of water, and the other on a table top side by side, in direct sunlight. Record the amount of tint that the lenses experience (remember, the lens on the table is the control, so compare the lens in the tank accordingly).

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