Remote sensing instruments use electromagnetic radiation to view the Earth. Electromagnetic rays are fast-moving waves of pure energy that are emitted by the Sun.
Each ray is a rippling electric and magnetic field and travels through space at a top speed of 300,000 kilometres a second, faster than anything else in the universe.
The Earth's atmosphere blocks, or absorbs, gamma rays, X-rays and ultraviolet rays; hence this energy is not used for detecting archaeological features. It also absorbs some infrared and microwaves. Radar systems use microwaves to determine the shape and roughness of the terrain, and archaeologists also use microwaves for geophysical prospection on the ground. All visible light passes through the Earth's atmosphere. Visible light is the only type of electromagnetic radiation that our eyes can detect. It's called visible for the simple reason that we can see this radiation, which covers a small range or 'spectrum' of wavelengths.