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10.10: Invisible Radiation

We take in the world through our eyes. Sight is arguably the most powerful and sophisticated sense; the one many of us feel we could not do without. Yet the visible spectrum that we see — the richness of the rainbow from red to blue — is just a tiny slice of an enormous array of types of radiation. There is an unseen universe waiting to be explored.


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Diagram of the Electromagnetic Spectrum. Click here for original source URL


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Johann Ritter. Click here for original source URL.

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William Herschel. Click here for original source URL.

English astronomer William Herschel opened the first chapter in the story of human discovery of invisible radiation. In 1800, he dispersed the Sun's rays with a prism and placed a thermometer beyond the red end of the spectrum. The temperature rose, showing that the thermometer had absorbed invisible solar radiation with a wavelength longer than that of red light. The next year, German chemist Johann Ritter created a spectrum in the same way and placed paper soaked with silver chloride beyond the violet end of the visible rays. The paper darkened, indicating it had absorbed invisible radiation with a wavelength shorter than the shortest wavelength of blue light. Like explorers, these scientists had traveled beyond the rainbow, measuring waves that the eye cannot see.

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Guglielmo Marconi. Click here for original source URL.

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Wilhelm Roentgen. Click here for original source URL.

Around the beginning of the 20th century, another pair of discoveries pried open the spectrum of radiation even further. In his darkened laboratory, Wilhelm Roentgen passed electricity through a tube filled with gas at much lower density than the air. To his surprise, a chemical-coated screen on the other side of the room glowed whenever he passed electricity through the tube. The discovery was accidental, but as any good scientist would have done, Roentgen used logic and further experimentation to try and understand his observation. Light could not be responsible; the room was darkened and the tube was encased in thick cardboard. When his hand passed between the tube and the screen, he was startled to see the bones in his hand, as if the flesh had been stripped away! Newspapers gave prominent coverage to this spectacular discovery. Roentgen had discovered a strange new form of high-energy radiation: X-rays. He was awarded the first Nobel Prize in physics. In the same year, 1895, young Guglielmo Marconi experimented with long-wavelength radio waves that traveled through space and walls and people unimpeded.

People considered the types of radiation studied by Roentgen and Marconi to be wonderful and mysterious. Today, we take them for granted. X-rays are one of the essential elements of modern medicine, and radio waves are the basis for worldwide communication. How can we use these waves exist that are much shorter and much longer than the waves of visible light to explore the invisible universe? The answer is that in the last fifty years, astronomers have increasingly learned about the universe using electromagnetic waves that are much shorter than and much longer than waves of light.