OAT Practice Test 12

Lenses That Respond to Light

¹Photochromic spectacle lenses darken in bright sunlight and clear again indoors, sparing the wearer from switching between ordinary glasses and sunglasses. The word photochromic describes any material whose color changes reversibly when it absorbs light. Although the effect seems simple, it depends on a balance between two competing processes, and understanding that balance explains both the strengths and the familiar frustrations of these lenses.

²The first commercially successful photochromic lenses, introduced in the 1960s, were made of glass containing a great many microscopic crystals of silver halide, the same light-sensitive compound used in photographic film. When ultraviolet light strikes a crystal, it frees electrons that convert some silver ions into tiny clusters of metallic silver. These clusters absorb visible light, and the lens darkens. In photographic film the change is made permanent by chemical processing. In a lens, however, the halogen atoms released by the light remain trapped in the surrounding glass, close to where they were formed. When the light is removed, they recombine with the silver, the clusters disappear, and the lens clears. Small amounts of copper added to the glass help this reverse reaction proceed.

³Most lenses sold today are plastic, and plastic cannot hold silver halide crystals in the same way. Instead, plastic photochromic lenses contain organic dye molecules dissolved in the lens material or in a thin layer near its surface. In their resting form, these molecules absorb only ultraviolet light and are therefore colorless. Absorbing an ultraviolet photon causes a ring of atoms within the molecule to break open, and the molecule rearranges into a flatter, more extended shape that absorbs visible light. The opened form is less stable than the closed one, and with time it snaps back to its original shape, restoring the clear state.

⁴In both kinds of lens, the darkening reaction is driven by light, while the reverse reaction, at least in the most common designs, is driven mainly by heat. At any moment, molecules or silver clusters are being switched on by ultraviolet light and switched off by thermal motion. Under steady conditions the two processes reach a balance, and the darkness of the lens reflects that balance rather than the brightness of the light alone.

⁵This explains why temperature matters so much. On a cold, bright winter day, the reverse reaction is slow, so the lens becomes very dark; on a hot summer beach, with equally strong sunlight, the faster reverse reaction keeps the lens noticeably lighter. The same principle governs how quickly lenses clear. A wearer who walks from bright snow into a warm building may wait several minutes for cold lenses to fade, whereas lenses that are already warm clear more quickly. Manufacturers try to choose dyes that darken strongly yet fade quickly, but the two goals pull in opposite directions: a molecule that returns readily to its clear form also tends to darken less in the sun.

⁶Another common complaint concerns driving. Most automobile windshields block nearly all of the ultraviolet light from the sun while transmitting most visible light. Because conventional photochromic lenses are activated by ultraviolet light, they darken only slightly inside a car, even on a bright day. Some newer designs use dyes that also respond to the shortest wavelengths of visible light, allowing partial darkening behind a windshield.

⁷Finally, photochromic dyes do not last indefinitely. Each switching cycle carries a small chance that a molecule will undergo an unwanted side reaction and permanently lose its ability to switch, a process called fatigue. Over years of use, plastic lenses may darken less fully or retain a slight residual tint. The silver halide system in glass lenses, by contrast, is regenerated with each cycle and is highly resistant to fatigue, one reason glass photochromic lenses were long valued despite being heavier than plastic ones.