For wearers of prescription eyeglasses, the development of “photochromic” lenses may seem like an answer to a fervent wish. These lenses exhibit “photochromism,” the mechanism by which they darken outdoors (when exposed to ultraviolet rays) and fade back to their initial transparent state indoors (or behind UV-blocking windows). This transformational process is possible because of the silver chloride or silver halide molecules embedded in the lenses, which change shape due to a chemical process when exposed to UV light. Their new molecular structure then absorbs portions of visible light, which causes the lenses to darken. The benefit for wearers of glasses with photochromic lenses is that they do not have to switch eyewear when going outdoors and back inside. No one wants to carry around two pairs of glasses. When you’re driving or walking during a sunny day, you no longer need to switch from your reading glasses to sunglasses. Because an automobile windshield blocks most, or all, UV light, photochromic lenses may not darken in cars and therefore may not be suitable as driving glasses.