Atmospheric Transmission--Good News and Bad News


This page has been translated into Swedish, thanks to Daniela Milton (June 2014).

It is not coincidence that our eyes are sensitive to optical light waves. Optical light can travel nearly unimpeded through our atmosphere, and so our eyes have adapted themselves to be sensitive at these wavelengths. For thousands of years, optical light was our only "window" on the Universe, and even then the primary "instrument" was the naked eye. When Galileo first turned his telescope skyward, he opened this window a little farther and saw many marvelous things for the first time. Yet, as is discussed elsewhere in these pages, optical light is a small portion of the entire range of electromagnetic radiation. How much more could we learn about the Universe if we could use the entire EM spectrum?

Most of the rest of the EM spectrum gets absorbed or blocked by the atmosphere, and it's a good thing, too! Just a tiny portion of ultraviolet light leaking through the atmosphere is enough to give us a painful sunburn if we aren't careful. Can you imagine what earth would be like if ultraviolet or X-ray light could get through the atmosphere? Suffice it to say that we wouldn't be around to "see" it--we would have long since been fried by our own sun! The atmosphere is "good news" from the perspective of life on earth. But the atmosphere is "bad news" if we want to see the Universe at these wavelengths.

This graphic shows schematically how the atmosphere lets some light through while blocking most kinds of light like X-ray, ultraviolet, and parts of the infrared. (Click on the graphic to see a larger, more readable version.)

Radio light was the first window after the optical one to be used for astronomy. This is because certain portions of the radio spectral region can pass through the atmosphere and be detected from cosmic sources. As a matter of fact, radio astronomy grew out of the development of radar techniques during World War II. As the first sensitive radio receivers were built, users noticed a persistent faint radio noise that they could not identify. It turns out this "noise" was due to cosmic sources of radio radiation being picked up by accident. Radio astronomy was born!

Certain portions of the infrared spectral region can get through the atmosphere, at least partially. Observatories at high altitude (above most of the water vapor in the atmosphere) can do quite well in certain small wavelength ranges. And special infrared telescopes have been flown on high-flying aircraft to open this window even further. However, many portions of the infrared spectrum get completely absorbed in the atmosphere.

While much can be learned from optical observations, there is also much unique information to be gained from observations in other wavelength ranges that are hidden by the curtain of our atmosphere. For astronomers to observe in most of the EM spectrum, we need to get telescopes in space. It has only been over the last 35 years that this has been possible, with rockets that can lift special telescopes above the atmospheric curtain. Telescopes have now been flown to observe across the entire spectrum! For astronomers, that's good news!

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Bill Blair (wpb@pha.jhu.edu)