The electromagnetic spectrum is the range of electromagnetic radiation from the shortest waves, called gamma rays, to the longest, called radio waves. EM waves include radio waves, microwaves, infrared radiation, ultraviolet rays, and X-rays.
Satellites can have many types of cameras or specials sensors that looks at the Earth. The different types of land cover on Earth -- water, trees, grass, rock, soil, cities, etc. -- act differently when sunlight strikes them. The different wavelengths of EM radiation from the Sun can be reflected, absorbed, or transmitted in various amounts by the different surfaces. This makes it possible to spot different substances based on their "spectral signatures."
Look at the example below based on a satellite image of Biloxi, Mississippi:
If we zoom in on the beach, the water and vegetation in this picture, we can check the spectral signature of each kind of land cover.
Beach Water Forest
Some types of light are reflected by surfaces like sand but absorbed by plants. The opposite may be true for other types of EM radiation. Remote sensing satellites carry many types of sensors and can pick up light from across the EM spectrum.
Because remote sensing satellites can see more that just visible light, they are especially useful for looking at plants and trees which give off infrared energy. This allows environmental scientists to distinguish between cities and forests and track the pace of development and environmental change.
So the electromagnetic spectrum is important because it lets scientists using special equipment see things that the normal eye wouldn't be able to see. The spectrum lets us view pictures of the Earth explore how the environment is changing.
http://observe.arc.nasa.gov/nasa/education/reference/emspec/emspectrum.html
http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html
http://rst.gsfc.nasa.gov/Intro/Part2_2.html
http://rst.gsfc.nasa.gov/Intro/Part2_5.html
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