Hello, For reasons I’ve read here in this forum, some material (let’s say PVC or paper) are not transparent to light. But light is an electromagnetic wave. If I lower the frequency, that material is now transparent, even if I cannot detect it with my eyes but must use special equipments, the same if I increase the frequency. It seems only in the bandwidth of visible light, that material in not transparent. Can some one tell me the reasons? Best regards.