I am a bit confused about my conceptual understanding of "light". As always I warn that I have only the simplest naive understanding of physics. In thinking about vision, especially color vision, I understand that color as such is not a real property of the external world. Nor for that matter is light and dark. So when we talk about light, we are really talking only about electromagnetic radiation (EMR) in a limited range of frequencies. As "light" and color are subjective inner properties assigned by our brains to the interaction between our eyes and EMR, I'm wondering exactly why our eyes/brains have evolved to use the limited range of frequencies that make up visible light. My question is simply, are there particular qualities of EMR that differ one from the other depending upon essential properties such as frequency or amplitude? A quick read about the electromagnetic effect suggests so, given that the threshold frequency appears to be visible light (I confess that seems to me a strange coincidence). My naive take about EM radiation is that its a sort of linear thing - waves/particles that differ largely only in frequency/wavelength/intensity, but clearly there is a lot more to it? Or do I misunderstand things here completely?