Why Do Those Glasses Darken Outside and Not in the Car?
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Certain molecules rearrange when they absorb ultraviolet light and rearrange back in the dark, and the specific wavelength that triggers them explains a familiar annoyance.
What happens in the material
The active substance exists in two stable forms with different structures and different colours. Absorbing a photon of the right energy, which is in the ultraviolet range, breaks or rearranges a bond and converts the molecule to the second form, which absorbs visible light and therefore appears dark. Left alone, thermal energy converts it back over seconds to minutes. The change is a genuine chemical rearrangement rather than a filter being applied, and the same molecules cycle back and forth indefinitely, though not without limit.
What the behaviour depends on
Several properties follow directly from the chemistry:
- •Ultraviolet triggers the change, so visible brightness alone does not
- •Darkening is fast and fading back is considerably slower
- •Cold slows the return, so lenses stay darker in winter
- •Heat speeds it, so they darken less on a hot bright day
- •Repeated cycling degrades the molecules over years
- •The clear state is never quite as clear as ordinary glass
Why they fail in a car
The familiar complaint has a precise explanation. Ordinary window glass blocks most of the ultraviolet range that triggers the reaction, and laminated windscreens block nearly all of it, so the molecules inside a car receive very little of what they respond to however bright the scene looks. Visible light passes through freely, so the driver experiences full glare with clear lenses. Manufacturers responded with variants tuned to respond to some visible wavelengths as well, which work behind glass at the cost of a slight permanent tint.
The related switching materials
Several families of material change appearance in response to something and grouping them prevents confusion. Thermochromic materials respond to temperature and appear in mugs, thermometer strips and kettle indicators. Electrochromic materials respond to an applied voltage and are used in aircraft windows and rear view mirrors that dim automatically. Piezochromic materials respond to pressure. Solvatochromic ones respond to the solvent around them and are used as indicators. The family here responds to light alone, which makes it self-powered and simultaneously impossible to control.
Where else the chemistry is used
The same reversible switching is applied well beyond eyewear. Windows that darken to reduce cooling load use related chemistry, though most commercial smart glazing uses an electrical trigger instead because it can be controlled. Novelty items that change colour in sunlight, including toys, nail varnish and clothing, use the same compounds. Ultraviolet indicator beads used in teaching and in sunscreen demonstrations are simply the material without a substrate. And research applications use similar molecules as switches in data storage and in controlling biological molecules with light.
The takeaway
Molecules that rearrange into a darker form on absorbing ultraviolet light and relax back in the dark darken quickly and fade slowly, more in cold weather and less in heat. Ordinary and laminated glass blocks the triggering wavelengths, which is why lenses stay clear in a car while the glare does not. Variants tuned to visible light fix that at the cost of a permanent tint.