The photochromic lenses work with certain molecules that are built into the lens. They are transparent when in light without UV rays. However, when UV rays hit the molecules, they react to them by changing forms. This occurrence causes the lenses to darken. The more UV rays hitting the molecules, the darker the lenses will become.
These lenses solve a problem for people who need to wear eyeglasses, and cannot wear sunglasses other than prescription ones. You will no longer have to carry two pairs of sunglasses, making things a lot easier. Most people will not carry more than one pair around sacrificing protection for their eyes. If you have a pair of photochromic sunglasses that are prescription you get to have the best of both worlds.
When driving, these sunglasses can be particularly useful. We tend to go inside and outside a lot when traveling so having one of these sunglasses can be indispensable. You will no longer have to stop to switch to sunglasses when the sky suddenly becomes brighter. For sports, this can be very useful too. Especially when playing sports outdoors, and the sunlight changes due to the clouds moving, it is great to have a pair of sunglasses that can react to that. If you are playing tennis or volleyball, for instance, there is no time to stop in the middle of a game to go switch out sunglasses-especially if you are playing in a competition.
This is preferable by a small number of people, but it doesn’t do as good a job as protecting than quality lenses. A choice between these would also depend on your eyes. If you are more sensitive to light, then it may be worth it to pay extra for photochromic sunglasses that darken more as it gets brighter outside.