Although air is almost perfectly transparent, it can still cast shadows via refraction. The key principle regarding refraction is that light is bent when the index of refraction Air and glass are different materials and have different indices of refraction. Light therefore bends when it goes from air into glass, such as at the surface of a glass lens. Refraction does not happen inside a glass lens because the material inside the lens is uniform. Refraction happens at the surface of a glass lens because that is the only place where the index of refraction differs. Uniform air itself cannot refract light and create shadows because the index of refraction does not differ anywhere. But, when different regions of air have different indices of refraction, the air can indeed bend light away from the forward direction and create a shadow.