Sometimes the simplest things turn out to be an incredible demonstration of the wisdom and planning built into the creation. It may seem simple to answer the question contained in the title of our article, but this is a highly complex issue and incredibly difficult to design. Light is made up of waves that have different frequencies. There are many different energies of light, but we only see a small percentage of them. Some energies like X-rays are too high for us to see. Some energies like radio waves are too low for us to see.
Objects can do one of three things to a certain energy of light. It can adsorb that energy, making the object opaque. Wood is opaque to human vision. An object can also alter the direction of a certain light wave, allowing the light to go through but to be altered as it does so. Light is refracted or diffracted by some materials like a diamond, allowing light to get through the material, but to be altered as it does so. It is also possible that a material will essentially do nothing to light making the material transparent--although very few materials do this.
To understand this, imagine electrons going around the nucleus of an atom in certain paths. The paths are actually energy levels so each path has a certain energy level. When the energy level of a certain electron in a certain path is the same as the energy level of the light going through it, the light is absorbed and its energy is converted into heat. Wood has electrons that are arranged in such a way that their energy is the same as the energy of visible light. This makes wood absorb visible light. Clear glass does not have electrons with energies that absorb visible light, but it does absorb ultraviolet--the energy of light that gives humans a suntan, and infrared which is radiant heat. If you add something to the glass, that will change the energies of light that are in the glass. Adding copper, for example, will cause the glass to have a blue color because copper absorbs energies in the red end of the spectrum but does not absorb blue. Chromium has electrons that absorb red and blue energies of light, so when it is added to glass the glass is green.
If our eyes were sensitive to radio waves, most solid objects would be invisible to us. Just as a radio can be listened to in a closed room, there would be nothing in the houses in which we live that would have electrons at the energies of radio waves so radio waves would go through just about everything as we would be essentially blind. Some animals, like rattlesnakes, can see in the infrared--so they can see heat coming from a mouse they are about to eat while in total darkness.
The complexity of this system that allows us to see those things that our bodies are not able to pass through and have the ability to not see things our bodies can pass through like air, is far more intricate than it appears. Some pretty incredible engineering had to be done to make us able to see through glass and not through wood.
--Reference: Popular Science, February, 2001, page 76.
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