I couldn't sleep the other night because I was thinking about photons and their ability to penetrate materials. It can be argued that humans are photon detectors. This is true in more than just the sense of visible light because photons don't just enter our pupils. As an example, our bodies can detect x-rays (our bones stop them). But we can't form an x-ray map of our surroundings, as we can with visible light, unless computers assist us. Imagine for a moment if our eyes were unable to detect visible light. For some reason we evolved to detect x-rays. If I were in a building, away from all windows, what would I see? Assume that all light sources (in this case the sources would emit x-rays) were turned off. Since x-rays have greater penetrating power than visible light, would I still see the Sun if I were inside a building? If one had eyes able to detect x-rays would there be a need to make artificial x-ray sources? But then I thought about much longer wavelengths. As you know, if you step outside of a building and shine a flashlight against the wall, the photons will not pass through the wall and be detected by any humans inside. Interestingly, I asked, "Why can radio waves pass through the wall?" Since radio waves have less energy than light waves, how are they able to penetrate materials that absorb and reflect visible light? I used to think that radio waves were made of photons, but I read recently that they are not. It just seems logical to assume that the carriers of electromagnetic radiation, the photons, waved at all wavelengths. Since radio waves are not made of photons (are they made of electrons?) why are they put on the same chart as visible light, ultraviolet, x-ray, gamma-ray, etc.?