Chromic Materials come in two main forms: Thermochromic Materials- which are materials that respond to a change in heat and Photochromic Materials- which respond to a change in light. The most popular use for Photochromic Materials is Transitions Sunglasses, which change colour and tint of the glasses according to the amount of light outside. The word photochromic actually comes from the Greek language with Photo being light and Chromo being colour, or basically changing colour in response to light. Modern photochromic lenses tend to be made of plastic and have carbon-based molecules called naphthopyrans which react to light. When ultra-violet rays strike these naphthopyrans, they subtly change their molecular structure. In this newly changed format, these lenses then get to absorb more light as it tries to pass by, and in doing so, the lenses darken. This process is similar to closing the blinds on your windows.