Question: why is h20 clear

Christian Wirtz answered on 15 Nov 2013:

Hi,

this is a very good question and brings us to the fundamentals of materials science!

Light carries energy. This energy is specific to the colour of the light. Red light has some energy and as you move across the rainbow spectrum, it carries more and more energy until you get to violet light which carries most energy in the visible spectrum. UV light carries even more energy and then you get to x-rays and gamma rays, all of which carry so much energy that they can burn you, which is what makes them so dangerous.
If a material is to be opaque, it has to absorb light and that energy from the light has to go somewhere. It commonly goes into giving more energy to the electrons which are promoted to higher energy levels. But there is a catch: These levels are very distinct energies and not just any energy will do, a phenomenon called “quantisation”. That’s kind of like taking an elevator: You only want to go to a specific floor, there’s no point in taking the elevator if it drops you halfway between two floors or doesn’t even have enough power to get you to the first floor. The spacing between these “floors” of energy depends on the material, sometimes they are very low and any light will do to get the electrons there (in that case the materials are often black as they absorb all light). Sometimes red light does not have enough energy to get the electrons to a specific level but green or violet light does. Then the red light is let through the material but the rest isn’t.
And sometimes even violet light does not have enough energy to get the electrons even to the first energy level, the first “floor”. That’s the case with H2O. The electrons have no use for the energy carried by the light, so they just let it pass. The same applies to glass, which is why your windows transmit light.