DISCOVERY

Sir Frederick William Herschel (1738-1822) was born in Hanover, Germany, and became well known as both a musician and an astronomer. He emigrated to England in 1757, and with his sister Caroline, constructed telescopes to survey the night sky. Their work resulted in several catalogs of double stars and nebulae. Herschel is perhaps most famous for his discovery of the planet Uranus in 1781, the first new planet found since antiquity.

In the year 1800, Herschel made another very important discovery. He was interested in learning how much heat passed through the different colored filters he used to observe the Sun and noticed that filters of different colors seemed to pass different levels of heat. Herschel thought that the colors themselves might contain different levels of heat, so he devised a clever experiment to investigate his hypothesis.

Herschel directed sunlight through a glass prism to create a spectrum - the "rainbow" created when light is divided into its colors - and measured the temperature of each color. He used three thermometers with blackened bulbs (to better absorb the heat) and placed one bulb in each color while the other two were placed beyond the spectrum as control samples. As he measured the temperatures of the violet, blue, green, yellow, orange and red light, he notice d that all of the colors had temperatures higher than the controls and that the temperature of the colors increased from the violet to the red part of the spectrum. After noticing this pattern, Herschel decided to measure the temperature just beyond the red portion of the spectrum in a region apparently devoid of sunlight. To his surprise, he found that this region had the highest temperature of all.

Herschel performed further experiments on what he called the "calorific rays" that existed beyond the red part of the spectrum and found that they were reflected, refracted, absorbed and transmitted just like visible light. What Sir William had discovered was a form of light  (or radiation) beyond red light. These "calorific rays" were later renamed infrared rays or infrared radiation (the prefix infra means `below'). Herschel's experiment was important not only because it led to the discovery of infrared light, but also because it was the first time that someone showed that there were forms of light that we cannot see with our eyes.

Today, infrared technology has many exciting and useful applications. In the field of infrared astronomy, new and fascinating discoveries are being made about the Universe. Medical infrared imaging is a very useful diagnostic tool. Infrared cameras are used for police and security work as well as in fire fighting and in the military. Infrared imaging is used to detect heat loss in buildings and in testing electronic systems. Infrared satellites have been used to monitor the Earth's weather, to study vegetation patterns, and to study geology and ocean temperatures. The year 2000 marks the 200th anniversary of Sir William Herschel's discovery of infrared. Our eyes are detectors which are designed to detect visible light waves (or visible radiation). Visible   light is one of the few types of radiation that can penetrate our atmosphere and be detected on the Earth's surface. As we have seen from the section on the discovery of infrared, there are forms of light (or radiation) which we cannot see. Actually we can only see a very small part of the entire range of radiation called the * electromagnetic spectrum . The electromagnetic spectrum includes gamma rays, X-rays, ultraviolet, visible, infrared, microwaves, and radio waves. The only difference between these different types of radiation is their wavelength or frequency. Wavelength increases and frequency (as well as energy and temperature) decreases from gamma rays to radio waves. All of these forms of radiation travel at the speed of light (186,000 miles or 300,000,000 meters per second in a vacuum). In addition to visible light, radio, some infrared and a very small amount of ultraviolet radiation also reaches the Earth's surface from space. Fortunately for us, our atmosphere blocks out the rest, much of which is very hazardous if not deadly for life on Earth.

Infrared radiation lies between the visible and microwave portions of the electromagnetic spectrum.   Thus infrared waves have wavelengths longer than visible and shorter than microwaves and have frequencies which are lower than visible and higher than microwaves. Near infrared refers to the part of the infrared spectrum that is closest to visible light and far infrared refers to the part that is closer to the microwave region.   The primary source of infrared radiation is heat or thermal radiation. Any object which has a temperature i.e. anything above absolute zero (-459.67 degrees Fahrenheit or -273.15 degrees Celsius or 0 degrees Kelvin), radiates in the infrared. Even objects that we think of as being very cold, such as an ice cube, emit infrared. When an object is not quite hot enough to radiate visible light, it will emit most of its energy in the infrared. For example, hot charcoal may not give off light but it does emit infrared radiation which we feel as heat. The warmer the object, the more infrared radiation it emits. Humans, at normal body temperature, radiate most strongly in the infrared at a wavelength of about 10 microns (A micron is the term commonly used in astronomy for a micrometer or one millionth of a meter).

We experience infrared radiation every day. The heat that we feel from sunlight, a fire, a radiator or a warm sidewalk is infrared. Although our eyes cannot see it, the nerves in our skin can feel it as heat. The temperature-sensitive nerve endings in your skin can detect the difference between your inside body temperature and your outside skin temperature. We also commonly use infrared rays when we operate a television remote.

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