Forms of Electromagnetic Energy

Every object with a temperature above absolute zero radiates energy as electromagnetic radiation. Unlike sound, this radiation requires no material medium; it propagates through the vacuum of space at 3.0 × 10⁸ meters per second. What distinguishes one form of electromagnetic energy from another is not its speed but its wavelength — the distance between successive wave crests — and its frequency, the number of crests passing a fixed point each second. Because the speed is constant, these two quantities are locked in an inverse relationship: as wavelength shortens, frequency necessarily rises.

Page 14 of the Earth Science Reference Tables arranges this continuum from wavelengths near 1 nanometer to microwaves approaching 1 meter. Two arrows beneath the diagram encode the fundamental rule of the spectrum. Energy increases toward the left, where wavelengths are short and frequencies are high; wavelength increases toward the right. Astronomers exploit this rule constantly. The energy carried by an individual photon is proportional to frequency, so an ultraviolet photon delivers far more energy to a molecule than an infrared photon of identical brightness.

The visible spectrum — roughly 4.0 × 10² to 7.0 × 10² nanometers — occupies a strikingly narrow slice of the chart, yet it dominates human perception because the Sun’s emission peaks there and because Earth’s atmosphere is largely transparent to it. Wavelengths outside this window behave very differently. Some pass through the atmosphere freely and reach the ground; others are absorbed almost entirely long before they arrive.

The atmosphere is not equally transparent to every form of electromagnetic energy, and human activity has altered its filtering properties. Stratospheric ozone absorbs most incoming ultraviolet radiation. Because ultraviolet photons carry enough energy to break chemical bonds, this absorption shields surface life from damage to DNA and living tissue. Beginning in the 1970s, chlorofluorocarbons released from refrigerants and aerosol propellants drifted into the stratosphere, where ultraviolet light itself split the molecules apart and freed chlorine atoms that destroyed ozone. The resulting thinning over Antarctica allowed measurably more ultraviolet radiation to reach the surface. International restrictions on these compounds have since lowered their concentrations, and ozone levels are slowly recovering.

A second filter operates at the opposite end of the chart. Earth’s surface, far cooler than the Sun, re-radiates absorbed energy as infrared radiation. Carbon dioxide, methane, and water vapor absorb these longer wavelengths and re-emit them in all directions, including back toward the ground. Combustion of fossil fuels has raised atmospheric carbon dioxide concentrations, increasing the fraction of outgoing infrared energy that is intercepted and warming the lower atmosphere.

Both cases illustrate the same principle. The effect of electromagnetic energy on an Earth system depends on which wavelengths that system is able to absorb. Engineers apply that principle deliberately: satellite sensors tuned to infrared wavelengths image storms in complete darkness, while sensors tuned to ultraviolet wavelengths measure the thickness of the ozone layer from orbit.