Energy–earth interaction is the fundamental process on which the science of remote sensing is based. Remote sensing involves the acquisition of information about the earth’s surface without direct physical contact, and this is made possible only because electromagnetic energy interacts with natural and man-made features on the earth. The primary source of energy for most remote sensing systems is the sun. Solar radiation travels through space and reaches the earth in the form of electromagnetic waves. When this energy reaches the atmosphere and the surface, a series of complex interactions take place which determine how different objects appear on remotely sensed images. The first stage of interaction occurs in the atmosphere. As electromagnetic energy passes through the atmosphere, it is affected by atmospheric particles, gases and water vapour. The energy may be scattered, absorbed or transmitted. Scattering takes place when particles such as dust, smoke or aerosols deflect the incoming radiation in different directions. This is the reason why the sky appears blue, as shorter wavelengths are scattered more strongly. Absorption occurs when certain atmospheric gases, such as ozone, carbon dioxide and water vapour, absorb specific wavelengths of radiation. Only