In this article we will learn about various concepts of electromagnetic spectrum.
Electromagnetic energy generated from a source (Sun) is regarded as a continuum of energy. All wavelengths of energy exist in the continuum. When the distribution of electromagnetic energy is plotted against wavelength or spectrum it is regarded as electromagnetic spectrum.
Electromagnetic spectrum can be defined as a plot showing the distribution of electromagnetic energy as per wavelength/frequency in ascending or descending order. Generally, in electromagnetic spectrum wavelength ranges from shorter wavelengths to longer wavelengths.
Based on wavelength, the spectrum is divided into several zones. Each zone is given a name based on their spectral behavior. These zones are not well defined in terms of distinguishing behavior from one zone to another( i.e., not strict division from one spectral zone to another). These should be used only for separation purpose.
Various important zones of electromagnetic spectrum:
Following sections describe various spectral zones of electromagnetic radiation and its properties.
Gamma waves are not available for performing remote sensing. It is absorbed by atmosphere before reaching the earth surface. The wave length of gamma radiation is less than 3×10-5micro meters.
Similar to Gamma Radiation, X-Rays are also absorbed by atmosphere and are not available for remote sensing.
Ultra violet radiation wavelength ranges from 0.03 to 0.3 micro meters. Most of this radiation is absorbed by ozone layer present in the atmosphere before reaching the atmosphere. Very less amount of energy coming into the atmosphere is scattered by constituents present in the atmosphere at higher levels. Hence, Ultraviolet radiation is also not available for remote sensing purpose.
Most of the optical remote sensing is performed using energy present in visible portion of electromagnetic spectrum. The wavelength ranges from 0.4 to 0.7 micrometers. Visible portion of electromagnetic spectrum is further subdivided into 3 zones. Blue portion is from 0.4 to 0.5 micrometers, green is from 0.5 to 0.6 micrometers and red is from 0.6 to 0.7 micrometers.
Vegetation appears in green color because vegetation reflects green portion of energy in electromagnetic spectrum.
Infrared portion of energy ranges from 0.7 to 100 micrometers. Temperature of an object or heat is regarded as the response of objects due to infra red radiation. Infrared energy constitutes of 3 zones in the electromagnetic spectrum. The portion from 0.7 to 1.3 µm wavelength is called near infrared (NIR) radiation, the portion from 1.3 to 3 µm is Shortwave Infrared and 3 of 100 µm is called Thermal Infrared radiation. Objects release their absorbed energy in thermal wavelength.
Microwaves have longer wavelength. The wavelength ranges from 1mm to 1 meter of the electromagnetic spectrum. Microwaves have the capability to pass through clouds. Hence, in many applications related to atmosphere in remote sensing microwave sensors are widely used.
However, the problem with microwaves is they possess less energy ( higher wavelength – lesser energy – energy laws). As a result more area has to be observed to get a detectable signal, and external source of energy is used in remote sensing. Active remote sensing makes use of microwaves mostly in other words.
The wavelength extends upto several hundred meters. Radio waves are widely used in communication and broad casting applications. The use of radio waves is normally controlled by government agencies.