In The Dark


Concepts Shown:

photochromic glass


photochromic lens (Corning)


Let students examine lens while discussing information. Why do people wear photochromic lenses? Ultraviolet and blue light possess wavelengths that can cause damage to the retina at high intensities. These intensities are caused by reflections from white surfaces (snow), glass, or chrome (bumper). Photochromic lenses block out 90% of UV and blue light.


Photochromic: Material whose absorption spectrum, or color, varies with the intensity of electromagnetic energy present. Making the lens: Regular glass is not photochromic. Salts of silver and halogens (chloride or bromide) are added to glass and heated to 1400 degrees C. silver halides precipitate out of solution if the glass object is held at 600 degrees C for fifteen minutes. This forms a sea of particles that are sensitive to irradiation. These particles are small enough (1-2 millionth of an inch) that they appear transparent when electromagnetic radiation is not intense. The photochromic glass has a density of 2.41 grams per cubic centimeter. Photochromic process: Step 1: Ultraviolet and short wavelength visible light start the darkening process. Electrons are excited by energy from light. The intensity of irradiation is dependent on the light source. Sunlight > Cloudy > Florescent > Incandescent Halogen loses an electron. hv + Cl- -> Clo + e- Step 2: The micro-crystals dissociate into silver particles which cluster together. These clusters absorb UV and blue light. e - + Ag+ � Ago Step 3: Chlorine holes (Clo) move quickly through the lattice. A step must slow the process of chlorine and silver reuniting. Copper is used for this purpose. The cuprous ions diffuse close to an active halogen atom. This traps the hole from wandering back and combining with a silver aggregate. Cu+ + Clo -> Cl- + Cu2+ Step 4: The glass clears when cuprous ions diffuse close to a silver aggregate. Diffusion of cuprous ions is dependent on temperature. The warmer it is, the faster they move. Ago + Cu2+ -> Ag+ + Cu+ Notes: The speed of motion in lattice is the following: Faster hole, electron > Ag+ > Cu+ >Cl- Slower This is the reason why fading takes longer than darkening. The elements involved in the process are slower moving. Comments: A UV light source will darken the lens inside a building. If the light source is available, a short demonstration of the darkening process can be given.


Marcia Muller

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