Seeing the invisible

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How can you study something you can't hear or feel or even see? That you can't see something doesn't mean you can't find ways to learn about it.

Monday, a special observatory was launched into orbit above Earth. The Space Infrared Telescope Facility (SIRTF) is designed to see the invisible and tell us more about the universe. What it actually "sees" is infrared radiation that we can't detect with the human eye. This is just one way scientists are exploring the universe and what's on Earth using infrared radiation.

What we usually call "light" is just one portion of the radiation that is being created or reflected all around us. This radiation takes the form of waves of different wavelengths, and the human eye sees only one small part of this electromagnetic spectrum (see illustration, right). The part we can see is called visible light, and includes all the colors of the rainbow, which combine to make white light. But beyond this visible light, there are other types of radiation.

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Infrared radiation has wavelengths too long for us to see, but we can feel the results, because they include heat. When the sun feels warm on your face, it isn't the light you see that has the warmth in it. That comes from the infrared radiation that comes along with the sun's visible light.

Any object that produces heat also produces infrared radiation. Even an ice cube. Ice may seem cold to us, but compared with the emptiness of outer space, ice is quite warm. It gives off infrared radiation.

So do objects in space that don't give off visible light, or whose light is blocked by dust. An infrared telescope can detect objects we can't see when we're looking just at the visible light that reaches Earth. This type of telescope can peer through vast clouds of dust and gas and look into the centers of galaxies.

To do this, though, the telescope has to be very cold. Otherwise, all it would detect is its own heat, instead of the faint infrared radiation from distant objects in space. And since Earth also emits infrared radiation, the new observatory will orbit the sun, traveling behind Earth at a distance. That way, the Earth's infrared radiation won't be as intense. SIRTF will also have a special heat shield to protect it from the heat of the sun and Earth. There it will stay cool and be able to detect and send us infrared images.

The colors we see, red, blue, green, are different wavelengths of visible light. Infrared comes in different "colors" or wavelengths, too. "Near" infrared radiation has a wavelength close to visible light, to the color red. This radiation isn't very warm. Your TV remote control sends a beam of this near infrared "light" to the TV. Far infrared has longer wavelengths, close to the wavelengths of microwaves. This is the type of radiation that we feel as heat. Fast-food restaurants sometimes use this type of infrared to keep hamburgers warm.

You even create infrared radiation of your own. You don't produce visible light yourself; you just reflect the light from the sun or another source. But your body does produce heat. In the infrared portion of the electromagnetic spectrum, you glow. Firefighters use this glow to help find people in smoke-filled rooms. They use special "glasses" that change the wavelength of infrared light to visible light.

This is also how infrared cameras work. It wouldn't help us very much for a telescope to detect infrared light out in space if we still couldn't see it. Special equipment "translates" the infrared light into wavelengths that we can see. You can program an infrared camera to use visible colors that you select to represent the different infrared wavelengths. Often bright colors such as red or yellow are used to show far infrared radiation, because it has the most heat.

If you look at a picture of a cat in infrared, the eyes will be the brightest part. The cat's fur keeps most of the heat inside, but the eyes let heat out, so they appear very "bright" in an infrared image. A picture of a dog will look dark at the nose, showing that dogs really do have cold noses. If you'd like to explore how different animals look in infrared, you can visit an "infrared zoo" at http://coolcosmos. ipac.caltech.edu/image_galleries/ir_zoo.

Scientists use this type of picture to learn more about animals. By looking at infrared images, researchers can see how well feathers, fur, or blubber insulate the animals or help them retain heat. They can tell coldblooded from warmblooded animals by how much heat they radiate. Using cameras or infrared "glasses," they can watch animals at night and learn about their behavior when it's too dark to see them in visible light wavelengths.

Infrared satellites also look back at the Earth to help us understand the weather. By looking at clouds in infrared, we can see more of the details that tell us how the clouds are structured. We can measure differences in ocean temperatures.

Infrared images can also help us study large areas of vegetation on the Earth. Healthy vegetation will appear differently in infrared, because the chlorophyll in plants (which also makes them appear green to us) reflects near infrared radiation from the sun. Viewing buildings in infrared helps determine where they are losing heat in winter.

Scientists are also looking for other ways infrared can help us see the invisible and learn about ourselves and our environment. A new system in Canada is trying to detect animals on the roads and warn motorists to watch out for them. Perhaps someday you will make a new discovery by looking at ordinary things in creative new ways.

To see more infrared images of people, animals, space, and Earth, visit http:// imagers.gsfc.nasa.gov/ems/infrared.html.

The electromagnetic spectrum

The electromagnetic spectrum is just another name for various types of radiation. Radiation is being created or reflected all around us, but we can only see a very small portion of it, called visible light. Infrared is another type of radiation. We can't see it, but we can sometimes feel it in the form of heat.

Radiation takes the form of waves, and each type has a different wavelength. A wavelength is the distance from a point on one wave to the same point on the next one. The shorter the wavelength, the greater the amount of energy that is transmitted by the waves.

Who discovered infrared radiation?

How do you discover something you can't see - when you don't even know it exists? In the case of infrared, as in some other scientific discoveries, it happened by accident.

Sir Frederick William Herschel, a German-born musician and astronomer, moved to England in 1757 and built his own telescopes to survey the night sky. In 1781 he discovered the planet Uranus.

Later he noticed that different amounts of heat seemed to pass through the different colored filters he was using to observe sunlight. He thought maybe each color had a different temperature. He used a glass prism to create a spectrum by dividing the sunlight into its colors. Then he measured the temperature of each color. He discovered that each color had a different temperature, moving from violet, the coolest, across the spectrum to red, which was the warmest. He decided to measure the spectrum just beyond red, and was surprised to find that this region had the highest temperature of all.

Herschel called this mysterious region of the spectrum "calorific rays," from the word "calorie," for heat. Later they became known as infrared, because "infra" means below, and this radiation is in wavelengths below those of visible light (see graphic).

Herschel conducted many experiments on these rays he had accidentally discovered. Today, scientists are still learning about the nature of infrared and the ways it can tell us more about our world and the universe.

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