Published: 10 November, 1999

Light waves allow us to see through solids

If solids, like glass and ice, are made of tightly packed molecules, how can we see through them?

To understand how light interacts with matter, it is important to remember that light is a wave. Just as the various notes you hear from a musical instrument correspond to acoustic waves of different oscillation frequencies, each color of light corresponds to a different frequency of an electromagnetic wave. Sunlight appears white since it is composed of a wide range of frequencies, whereas a laser produces light over an extremely narrow range of frequencies and thus the light appears as a single color.

When light interacts with a material, light energy can be transferred to the electrons in the material resulting in the absorption of the light beam. Depending on the atoms and molecules that compose the material, certain bands of light frequencies are strongly absorbed. However, there are frequency gaps between these bands within which no energy transfer can occur, and the light is completely transmitted through the material.

What makes materials like glass so important to us is that it transmits light over the range of frequencies at which our eyes can see. On the other hand, since glass absorbs ultraviolet light, you cannot get a tan from the sunlight that passes through a window. You can dramatically change the transmission properties of a material by changing its composition slightly. For example, stained glass is made by adding small amounts of different metals to the glass as it is being made. For example, adding oxidized copper narrows the transmission gap of the glass so that only red light can pass through it.