Reading ozone’s signature in sunlight
Sunlight reaching the ground has passed through the atmosphere, and gases along the way remove particular wavelengths from it. A spectrograph separates light into its component wavelengths, much as a prism makes a rainbow. In the early twentieth century, French physicists Charles Fabry and Henri Buisson examined the Sun’s ultraviolet spectrum and found absorption matching ozone’s known laboratory signature.
The key was the pattern: ozone absorbs ultraviolet light at specific wavelengths, leaving corresponding gaps in the sunlight measured at Earth’s surface. Those gaps showed that ozone was not merely present near the ground, where it had already been identified, but also existed high above it.
An atmospheric layer, not a visible sheet
The “ozone layer” is not a sharply edged band or a colored feature in the sky. It is a region of the stratosphere—the atmospheric layer above most weather—in which ozone is more concentrated than in the air below. Its molecules are spread through a vast volume of thin air, so the layer is invisible to the eye.
Ozone’s ultraviolet absorption has a powerful consequence: it reduces the amount of damaging ultraviolet radiation that reaches the surface. The same absorption that made ozone detectable also helps explain why its location matters. Scientists could infer an unseen part of the atmosphere from the light it removed—and that invisible chemical signature revealed a natural filter between the Sun and life on Earth.
