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Space age technology contains a key to increased knowledge about the Earth's resources; this key is remote sensing detecting the nature or condition of something without actually touching it. An early and still most useful form of remote sensing is photography which records the
scene, as man sees it, on film sensitive to that part of the electromagnetic spectrum called visible energy. Electromagnetic energy travels in waves of various lengths; most are invisible to the human eye. Wavelengths progressively longer than those that the eye can see are infrared and microwave. Wavelengths progressively shorter than the eye can see are ultraviolet, X-rays, and gamma rays.