I love whiteboards there quite

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I just got an eye exam

It was quite the SPECTICAL

Exactly a year back on this very day, we explored how polarized sunglasses work and why Pilots prefer NOT to use them.

What about those with power prescriptions you ask ?

These days you can find a lot of people even with power prescriptions sporting glasses/lenses that turn dark when exposed to sunlight. These are known as Photochromic lenses. 

Let’s do a quick demo:

This is a photochromic lens getting exposed to 395-400 nm light turning dark as a result.

(Better quality video here)

How does it do that ?

To learn how this is achieved it must be understood that there are these class of reactions called ‘Reversible reactions’

A and B can react to form C and D or, in the reverse reaction, C and D can react to form A and B. 

                    Example of reversible reaction of Bismuth Chloride

In the case of Bismuth Chloride, sunlight acts as a mediator to change it from a transparent solution to a dark colored solution and shaking in air brings it back.

In the case of Photochromic lenses, an organic dye is used that darkens under UV and lightens under its absence

                                                  Source                              

Alternate reality: Photo-degradation

If you ever have had the unfortunate experience of leaving your plastic in the sun or wondered why the colors of your clothes fade away, then the technical term to express your misery is Photo-degradation.

This is the other extreme of a reversible reaction where UV light from the sun break break down the chemical bonds in the plastic and clothes (called chromophores) and eventually leads to fading of the colors in the object

The catch

As any user of one of these Photochromic lenses will tell you, there are some aspects of these lenses that still need to be improved

-  The chemical reaction rate depends on the temperature of the surrounding. This means that on a sunny day if you are in a really warm climate, the lenses will adapt faster than if you live in a cold climate

- Most car windshields block UV light to some extent and therefore these lenses are not effective when you are inside the car.

- They do degrade after a while. Most of lenses do degrade having been used after a couple of years and need to be changed.

But having said that Photochromic lens are indeed a really cool piece of technology.

Have a great day!