When a photochromic lens is exposed to ultraviolet light, trillions of photochromic molecules in the lens begin to change the structure. This reaction is responsible for the darkening of the lens.  When the lenses are out of the sunlight, they become clear again

All lens-adapting lenses use photochromic molecules; the strength of branding technology lies in our unique patented formula. Each formula is integrated into the surface of the lens. These molecules are recalibrated continuously and steadily, so you get the best amount of light whether you're in bright sunlight, under clouds, or indoors. Photochromic lenses are specially treated to turn dark when exposed to UV light. 

ABBE Glasses Transitions® brand of photochromic lenses are in 1.50, 1.56, 1.61, and 1.67 lenses, for both single-vision and progressive glasses. The 1.50 Photochromic Transitions® - Brown and Gray is priced at cool price

Transitions® Signature™ VII 1.50 Photochromic Brown basic UV reactive CR-39 plastic lens.
Features:
• Suitable for spherical power of +8.00/-12.00 or below with cylindrical power +/-4.00 or below.
• Photochromic lens darkens to gray outdoors and turn clear indoors.
• May not darken inside a car or near windows since most windshields/windows block UV rays needed to darken the lenses.
• Anti-scratch hard coating included.
• Photochromic lenses may develop a slight base tint after long-term use.