Objects and surfaces absorb or reflect electromagnetic radiation of various wavelengths, including visible light and invisible bands. When we look at an object, the colours we perceive depend on the reflected wavelengths. A white object reflects all wavelengths. A blue object reflects mostly light in the blue spectrum, absorbing other wavelengths.
While the human eye can only perceive visible light, satellites are equipped with sensors which can detect radiation in the invisible spectrum (e.g. radio waves, microwaves, infrared). Some satellites actively send radio waves or laser pulses to Earth and record how they are reflected back. Such raw data is processed and colour codes are attributed to different surface properties. The resulting images allow the user to visualise information about, for instance, types of vegetation, micro-particles in the atmosphere, seawater salinity, algae blooms, burnt land, variations in sea or river levels, landslides and other properties.
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