Because people haven't really explained the specific mechanisms: You can think of a material as a bunch of simple harmonic oscillators, which are driven by an

Because people haven't really explained the specific mechanisms: You can think of a material as a bunch of simple harmonic oscillators, which are driven by an applied field (electric or magnetic). These oscillators have different natural frequencies, and so couple to different wavelengths. For instance, if you play a loud note next to a piano, you can see the corresponding piano string start to vibrate, but the others may not. The interaction between these oscillators and the wave are what lead to the dielectric constant (and therefore the slowing of waves), as well as absorption (which can be thought of as the imaginary component of the dielectric constant). In a real material, these "oscillators" are really any method of energy storage that can couple to the motion of charges (i.e. current). These include, but are not limited to: - rotations (in a gas or liquid) - vibrations (in any state) - electronic transitions - electronic movement (in the case of a metal) - displacement (in any state) In a single molecule, many of these mechanisms would have discrete natural frequencies. But in a solid or liquid, interactions lead to a continuous band structure (especially for things like vibrations). For water specifically, the below visible range is quickly absorbed by vibrational and rotational energy modes, while the high end of the UV range is absorbed by electronic transitions. Other materials have similar sweet spots for transmission, but at different frequencies. For instance, materials like indium tin oxide (ITO) are designed to be conductive, but not at the high frequencies of visible light, making them transparent. As another example, metals are reflective below their plasma frequency (related to the speed the 'electron sea' can move at), and transparent above (X-rays operate in this region of transparency). If you want more information, I can recommend "Optical Properties of Solids" by Mark Fox.