Discovery Of Visible Light

You're reading Discovery Of Visible Light, posted on Wednesday, February 14th, 2007 at 9:40 am in Space Technology, on BrainBloggers at the Astronomy blog. More after the jump.

In 1666, Isaac Newton took the first step toward trying to understand visible light when he passed sunlight through a rainbow prism and watched it break into a kaleidoscope of colors. Just like the wavelength differences between various parts of the electromagnetic spectrum, the array of colors comprising visible light are made of waves that have different lengths between successive crests or troughs. Red waves are the longest and violet waves are the shortest.

William Herschel, the discoverer of Uranus, was the first to explore the region beyond the rainbow of visible light. In the year of 1800, he reported on experiments to determine the heating power of different colors, in which he let a spectrum of light fall on a set of thermometers. To his amazement, he found that the greatest heat to be found was produced off the red end of the spectrum where nothing was visible to anyone. “Radiant heat,” he proclaimed, “will at least partly consist, if I may be permitted the expression, of invisible light.” Herschel had discovered infrared radiation.

In the mid-19th century, astrophotographers began to explore the other end of the spectrum. Photographic emulsions are particularly sensitive to shorter wavelengths of light, blue and violet. Astronomers discovered that stars and nebulae radiate beyond violet light, hence the term “ultraviolet” was created. Physicists completed the picture in the late 19th and early 20th centuries with the discovery of radio waves, ultraviolet light, X rays, and gamma rays.

Astronomers came to realize that many astronomical bodies radiate little or no visible light at all and remain hidden without full spectral exploration. Trying to understand the universe through visible light alone is like listening to a Beethoven symphony and hearing only the cellos. Only by studying images and spectra taken across the entire electromagnetic spectrum can astronomers appreciate the full scope of the universe.

In the 1930s, Karl Jansky, an engineer for Bell Labs, took the very first step in opening up the entire electromagnetic spectrum when he discovered radio waves from deep space using a simple antenna. Radar technology developed during World War II eventually led to the radio telescopes of today. Radio telescopes “hear” nothing. Their mission is to map where radio waves are coming from so astronomers can learn about the radiating bodies’ physical conditions. The way in which radio telescopes work is similar to optical (visible light) telescopes. They have large, curved collecting surfaces that reflect incoming radio photons to amplifiers and receivers.

Because radio waves have long wavelengths, they are not good at resolving small structures. To maximize resolution, astronomers build very large radio telescopes up to 1,000 feet (300 meters) across. Still, these telescopes have about the resolution of the human eye. Astronomers electronically coordinate individual radio telescopes hundreds of miles apart or synchronize their observations with clocks to construct “virtual” telescopes of continental size. Such radio interferometers can resolve details far sharper than anything current optical telescopes can see.

After developing radio astronomy in the mid-20th century, astronomers turned their attention to the rest of the electromagnetic spectrum. After languishing since the days of Herschel, the field of infrared astronomy has come alive in the last 20 years with the advent of good detectors that can be fitted to optical telescopes. More recently, astronomers bridged the gap between radio and infrared, millimeter waves and microwaves, and have now mastered the entire long-wave realm of the spectrum. Herschel would have been proud.