Alternative Assessment

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2013 Alternative Assessment Proposal

(i) Application of chemistry/ chemical phenomenon

Transition glasses have photochromic lenses which give eyeglasses the ability to change their tint level and adjust to the light that is available. They were designed to protect the eyes from harmful ultraviolet (UV) radiation and can be considered sunblock for the eyes. These lenses are clear indoors and darken outdoors for greater comfort in the sun. The photochromic lenses contain millions of molecules of substances such as silver chloride or silver halide which respond to ultra-violet radiation. The molecules are transparent to visible light in the absence of UV light, so it remains clear under artificial lighting. However, when exposed to UV rays like 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. Hence, the stronger and brighter the UV light, the darker the lens will be. When you go indoors and out of the UV light, a different chemical reaction takes place. The absence of the UV radiation causes the molecules to "snap back" to their original shape, resulting in the loss of their light absorbing properties. In both directions, the entire process happens very rapidly.

(ii) Topics and concepts involved

(iii) Risk assessment

|Possible hazards / Incidents |Precautions | |Over-exposure to the sun (UV radiation) during filming |Avoid looking directly at the sun, even with the photochromic glasses on | | |Hydrate yourself before filming | |...
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