Do the photochromic lenses fascinate you too? Well, for me they have always been a mysterious topic. The way the glasses acquire the tint when exposed to the sunlight, has always made me wonder, as to why the transition takes place. As a kid, owning them and showing them off to my friends had always been a matter of pride for me. Like me, there must be many who want to understand how the photochromic glasses work. So let us check out the truth behind the photochromic conundrum.
The word “photochromic” is derived from two Greek words “Photos” which means Light and “Chroma” which means Colour. Thus, photochromic means the something that changes its colour in response to the Light.
These were originally developed in the 1960s by Donald Stookey who worked as a chemist at Corning Glass Works. Soon after Stookey patented the material, Roger Araujo, another Corning chemist, developed the first photochromic lenses.
Photochromic lenses are made up of three types of material that are glass, polycarbonate or plastic. The photochromic lenses made up of glass have microcrystalline Silver halides (AgX-most commonly chlorine) embedded in the glass.
The plastic versions were based on based on thin films of organic compounds like oxazines i.e pyridobenzoxazines, naphthopyrans or indenonaphthopyrans in them.
The transition depends on the principle of equilibrium.
Chemical equilibrium – It is the state in which both reactants and products are present in concentrations that have no further tendency to change with time. Usually, this state results when the forward reaction proceeds at the same rate as the reverse reaction.
The exposure to the UV rays disturbs the equilibrium and hence the tint appears.
Most commonly Silver I chloride is used. On exposure the silver gains an electron from chloride to become silver metal, and gets the ability to absorb visible light and appear darker. The silver metal is mainly responsible for the dark tint or shade in the lenses.
The organic films of plastic photochromic lenses are more susceptible to degradation than the silver halides used in glass, even when the plastic lenses are lighter and thinner than their glass counterparts.
i. UV exposure – More the exposure more will be the darkening
ii. Thickness – Thicker glass has a darker tint as compared to the thinner ones.
iii. Temperature – The darkening is inversely proportional to the temperature that is more the temperature less will be the darkening.
These colour changing lenses provide the correct amount of protection for the varying light conditions and absorb 100% UV radiation that can otherwise damage the eyes.
However, they do not adjust immediately and take a few minutes to change from light to dark or vice versa. They don’t always work effectively in cars because windscreens naturally screen out most of the ultraviolet light and thus they do not acquire the tint.
Now that you know the reason behind the magical Photochromic glasses, you can spread the knowledge amongst your friends who are unaware of it. So, stay curious and keep learning.
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