Why We See Through Glass and Not Wood

How was 'Superman' able to see through walls?

Sometimes the simplest things turn out to be a demonstration of the wisdom and planning built into the creation. Just being able to see through some objects and not others is a highly complex issue and an incredibly difficult design.

Light is a form of electro-magnetic wave energy. Light waves of different colors have different frequencies (energy levels), and they are only a small portion of the entire spectrum of electro-magnetic frequencies. Some electro-magnetic wave frequencies are higher than the frequencies of light and we can’t see them -- like X-rays. Some are too low for us to see -- like radio waves.

Different materials can have one of three effects on electro-magnetic waves at different frequencies. They can absorb the waves, they can alter the direction of the waves, or they can allow the waves to pass through without alteration. If a material absorbs the waves in the frequencies of visible light, we can’t see through it, and we say that it is opaque -- like wood. If it allows visible light frequencies to pass though, we say that it is transparent – like glass. However, even glass is not completely transparent, it does absorb some light and alters the direction of the light to some degree.

When we say that an object is opaque or transparent, we are really saying that it is opaque or transparent to electro-magnetic waves in the frequency of visible light. However, wood is transparent to electro-magnetic waves in the radio frequency range. For that reason we can listen to radio or use our cell phones inside our houses. In fact, if our eyes were sensitive to radio waves, we would be able to see through most solid objects! But then we wouldn’t be able to see the things we really need to see. It wouldn’t be much fun to bump into an invisible door! However, we could have Superman’s “X-ray vision” if we could see electro-magnetic waves that are outside the frequencies we call visible light.

To understand how visible light can pass through some materials, we can think of the atoms in that material containing electrons with different energy levels. When the energy level of a certain electron in the atom 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. For this reason wood absorbs visible light.

Clear glass does not have as many electrons with energy levels that absorb visible light. However, it does absorb frequencies slightly higher than visible light (ultraviolet) and slightly below visible light (infrared). If you add certain materials to the glass you can change the energy levels of the electrons in the glass. For example, adding copper will cause the glass to absorb light in the red end of the visible spectrum so that the light passing through will tend to look blue. Adding chromium to the glass will cause it to absorb both red and blue energies of light. When you look through the glass, objects will appear green since it is mostly the green light that is passing through.

The complexity of this system allows us to see those things we need to see, like solid objects that our bodies cannot pass through. It also allows our vision not to be blocked by things that we can pass through, like the air or water. And it allows us to see through some solid objects which can protect us, like glass windows in our homes and cars.

It took some pretty incredible engineering to make us see through glass and not through wood.