When looking for synonyms for the phrase "impervious to light," a few options come to mind. The first is "impenetrable." This word suggests that no light can pass through a surface or object. Another option is "opaque." This word describes something that blocks light from passing through, making the surface or object completely dark. A third option is "lightproof," which specifically refers to something that is designed to block light. Lastly, "light-tight" can be used to describe something that completely blocks out light and prevents it from passing through. All of these synonyms convey the idea of something being completely resistant or unresponsive to light.