Every material absorbs some light energy. The amount of absorption depends on the wavelength and the material. A thin window of ordinary glass absorbs little visible light, so it looks transparent to the eye. The paper this book is printed on absorbs much more visible light, so it looks opaque. (You can read these words because the blank paper reflects more light than the ink, which absorbs most light striking it and reflects little.) The amount of absorption can vary greatly with wavelength. The clearest glass is quite opaque
at an infrared wavelength of 10 μm. Air absorbs so strongly at short ultraviolet wavelengths that scientists call wavelengths shorter than about 0.2 μm the vacuum ultraviolet because only a vacuum transmits them.
Absorption depends very strongly on the composition of a substance. Some materials absorb light very strongly at wavelengths where others are quite transparent. For glass, this means that adding small amounts of certain impurities can dramatically increase absorption at wavelengths where glass is otherwise transparent. Removing such impurities is crucial for making the extremely transparent fibers used for communications. Typically, absorption is plotted as a function of wavelength. Some absorption peaks can look quite
narrow because the material absorbs light in only a narrow range of wavelengths; others spread across a wider range.
Absorption is uniform. The same amount of the same material always absorbs the same fraction of light at the same wavelength. If you have three blocks of the same type of glass, each 1-centimeter thick, all three will absorb the same fraction of the light passing through them.
Absorption also is cumulative, so it depends on the total amount of material the light passes through. That means a material absorbs the same fraction of the light for each unit length. If the absorption is 1% per centimeter, it absorbs 1% of the light in the first centimeter, and 1% of the remaining light the next centimeter, and so on. If the only thing affecting light is absorption, the fraction of light absorbed per unit length is a , and the total length is D, the fraction of light remaining after a distance D is
In our example, this means that after passing through 1 m (100 cm) of glass, the fraction of light remaining would be