What is a Transitions Lens?

Transitions Lenses are Photochromic lenses that are clear indoors and get dark outside. The transitions lenses get activated with direct exposure to UV rays. They are available in several colors, but the most popular colors are:
1. Grey
2. Brown

The Transitions lenses block 100% of the harmful UV Rays and can be very impact resistant when purchased in polycarbonate or trivex materials.

How Do Transitions Lenses Work?

It is a fascinated question that my son asked me while having dinner the other night.  The answer is:
An important fact to know is that there is a thin portion of the lens that is injected with the organic material that gets activated with UV rays.  The entire lens does not have those activating properties.  The front surface of the lens that is spread evenly across the surface area is the only part of the lens that gets dark. 
It is also good to know that how dark the lens gets when exposed to direct UV rays is inversely correlated to outside temperature.  
Example: 
The Transitions lens will get darker at colder temperatures as opposed to when it is exposed to UV rays at 100 degrees fahrenheit.  It is the biggest complaint from people that invest in the purchase of these lenses with the expectation that the lens will replace their sunglasses.  
I always like to say to people that transisions lenses are great, but do not replace the sunglasses because the sunglasses I recommend have polarized lenses.
Keep that in mind when deciding if Transitions Lenses (photochromic lenses) will solve the vision problem you are having.

How Long Do Transition Lenses Last?
Under normal circumstances, which we tend to refer to as "normal wear-and-tear" would be two years.  Depending on the conditions of use, they will fatigue around the two year mark.  I've seen the transitions lenses last much longer than two years, by the way.  Fatigue means they remain slightly activated and don't return to almost clear status as when they were newer. 

Final Thoughts:
Transitions or photochromic lenses are great to have in your wardrobe of eyewear.  They have a purpose, but what they don't do is they don't completely replace your sunglasses.  
So make sure you still have a pair of polarized sunglasses that protect you against the sun's harmful rays and filter light in a way that they eliminate all light coming in from oblique angles.  This is what makes polarized lenses superior to transitions lenses outdoors.  
Transition lenses are great and you should have a pair.