Hi guys, I am too old to remember basic knowledge about electromagnetic waves :D I am looking for a combination of an electromagnetic wave and a material such that the wave can pass through human tissues, but it is blocked by the material. All I know is that X-ray/gamma can penetrate human body but is absorbed by metals/lead. Are there any such other combination which are less harmful than X-ray. I have gone through Wikipedia for few hours but still have not found the answer yet. Could you please help me. Thank a lots.