Impact resistant does not mean impact proof. Any lens can break with enough force. Polycarbonate lenses are perfect for children, as well as for sports-involved teens or adults with active lifestyles. Also, some manufacturing occupations may demand impact resistance as a safety feature. Polycarbonate lenses are also thinner and lighter than conventional plastic or glass lenses.
Photochromic lenses have an adjustable tint that changes with the amount of available light.
In some ways, yes. As photochromic lenses darken they reduce glare that can contribute to eye strain. For years, photochromic lenses have been made of glass. In fact, one of the most popular glass photochromics is the SOLA PhotoGray Extra lenses. However, in recent years, plastic photochromic lenses have been developed.