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Seeing the invisible

By Sharon J. Huntington / August 26, 2003

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.

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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.

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.