To be honest, I really don't understand this at all. If we take cardboard for example, obviously gamma rays can penetrate cardboard because the light has such a high energy, and obviously radio waves can penetrate cardboard because a cell phone will work even when entirely enclosed in cardboard. So high and low energy photons will pass through cardboard, but I cannot see through it, so it seems that all (or nearly all) visible light will be absorbed in the cardboard. I suppose that my hypothesis for this would be that the sun emits a large amount of light in the visible spectrum, so our eyes have evolved to detect it, and plants have evolved to absorb this same spectrum of light for photosynthesis (both plants and humans thus evolving to best utilize the EM radiation given off by the sun). Since cardboard is made from plant material, it is thus specifically good at absorbing visible light, while allowing other wavelengths to pass through. I have no idea if that is true, but even if it is, it only answers my question for plant-based material... I still have no idea why glass can hold in heat, yet visible light and radio waves may still pass through.


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