The human eye can only see optical or visible light.  Light comes in many other colors -- radio, infrared, ultraviolet, X-ray and gamma-ray.  Astronomers can get more information about stars and other distant cosmic phenomena by using instruments and telescopes which can detect these different types of "invisible" light.

Infrared light is light which is too red for human eyes to see.  Humans and other warm-blooded animals glow in infrared light because they are warm.  The military uses infrared cameras to search for people at night, when they can't see them in reflected optical light. Another property of infrared light is that it can be used to see through dust in our galaxy, the Milky Way.  Astronomers use infrared cameras to see objects like quasars which can be hidden by dust.

X-ray light is light which is too blue for human eyes to see.  X-rays can penetrate your skin and muscles, which is why doctors use them to look at your bones.  The Earth's atmosphere blocks, or is opaque to, X-ray light, so astronomers put telescopes that detect X-rays in space.  They discovered that the Sun emits X-rays.

The Chandra Observatory and the Hubble Space Telescope are telescopes in space.  These telescopes are are detecting sources of high energy X-ray billions of light years away from Earth.  X-ray images of quasars, neutron stars and supernova explosions are providing new information about the universe.

Investigation One: Behavior of Invisible Light

You probably have an infrared light source and infrared detector in your house -- the remote control of your television.  When you press a button on your remote, it sends a signal in infrared light to your TV.  The signal is invisible to you, but the TV can "see" it.  The set turns on or off, the volume goes up or down, and the channels change.  Experiment with invisible light and compare its behavior to that of visible light.

Materials:

• flashlight
• TV remote control and TV
• A variety of opaque and transparent materials such as: clear plastic wrap, clear plastic bag, glass of water, plexiglass, black paper, plastic grocery bag, cardboard, piece of wood, paper bag, newspaper, etc.

Procedure:
1.  Stand about 6' away from the TV.  Turn on the flashlight and point the beam at the TV.
2.  Place an item between the flashlight and the TV.  Does the flashlight beam shines through the material? Repeat with the other items sorting them into two piles: those that are transparent, or let light through, and those which are opaque, or block, optical light.
3.  Point the remote at the TV.  Turn the TV on and off to test it.
4.  Place an item between the remote and the TV.  See if you can still turn on the TV.  If you can, then the item is transparent to infrared light;  If you cannot, the item is opaque to infrared light.
5.  Test each item to again to find which ones are transparent and which are opaque to infrared light.
6.  Put the items into the following groups:
        Opaque to optical light but transparent to infrared light.
        Opaque to optical light and infrared light.
        Transparent to optical light and infrared light.
        Transparent to optical light but opaque to infrared light.  (Hint -- try a thick piece of lucite.)

Questions:
* How would you use this information to design a window for your house or your car?
* What would use for light sources if you could only see infrared light?
* Why can't you use an infrared detector to find someone lost in rainy weather?

Extensions:

• The eyes of all animals on Earth are adapted to be able to see the visible part of the spectrum.  Find out how rattlenakes and other reptiles detect infrared radiation.
• Make a list of the ways infrared light is used in your home and by firefighters, policemen, meteorologists, doctors, and scientists.

Investigation Two: Reflecting infrared light

Mirrors reflect optical light.  Find out if infrared light can be reflected by a mirror.

Materials:

• small mirror
• flashlight
• TV remote control and TV

Questions:
* What conclusions can you draw about the path of an infrared light beam?
* Each color in an infrared image shows a different temperature.   Would you expect an infrared image of a person to have one or more colors?  Why?
*  Why do you think it is difficult to reflect X-ray light?