Opaque, Transparent, and Translucent Materials

Is Earth’s atmosphere transparent to infrared and ultraviolet radiation as well as to light?

Oxygen and nitrogen, the two principal components of our atmosphere, are transparent to light and to most infrared (IR) and ultraviolet (UV) wavelengths. Infrared from the sun helps warm Earth and long-wavelength ultraviolet is necessary for our bodies to produce vitamin D. Too much UV, however, harms the skin, and may damage our DNA. A high-altitude layer of ozone in our atmosphere protects Earth from all but a small fraction of the UV radiated by the sun. Ozone molecules have three oxygen atoms as opposed to the two in oxygen gas. In recent years large and growing holes in this protective layer have developed due to the emission of chlorofluorocarbon (CFC) molecules from things such as escaped gases from refrigerants and aerosol propellants. Because these gases are no longer being manufactured and existing stocks are gradually being replaced, the the growth of the holes should eventually stop.

Carbon dioxide, methane, and water vapor are transparent to light and short-wavelength infrared radiation, but are opaque to the long-wavelength infrared emitted by warm objects. They are called greenhouse gases. These gases pass the IR rays that warm Earth but reflect the IR rays emitted by the warm Earth back to the ground. Thus they act as an insulating blanket for Earth. Over the past decades the amount of carbon dioxide in the atmosphere has increased dramatically. Much of the increase is due to human activity. This increase is likely to lead to a warmer Earth that could shift weather patterns, disrupting food production, causing shifts in locations of forests and animals, and, by melting polar ice, increasing sea levels. The degree of warming and its impact on Earth and humans is under intensive investigation.


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