In the previous article, we have defined what is remote sensing and what are various constituents or stages of remote sensing. In this article, you are going to learn what is electromagnetic radiation and its importance. You will also understanding electromagnetic radiation properties and different laws that characterize electromagnetic radiation.
Electromagnetic radiation is one of the most important component of remote sensing without which remote sensing is very much difficult.
Electromagnetic radiation can be defined as a form of energy produced by electric and magnetic fields which are moving in mutually perpendicular direction. The electric and magnetic fields are created by a source of energy.
Electric field travels in a direction perpendicular to the direction of travel and changes its magnitude. Magnetic field also travels in perpendicular to the direction of travel and electric field. Irrespective of intensity of source, magnetic and electric fields magnitude, electromagnetic energy propagates at velocity of light.
What are characteristics of Electromagnetic Radiation?
All electromagnetic energy travels in the form of electromagnetic waves. Following are 2 most importance characteristics of electromagnetic radiation.
The length of one complete wave measured in the electromagnetic radiation is called wavelength. It can be defined as distance from one wave crest to another (or) distance measured from one wave trough to another.
Wavelength is measured in units of length. In case of electromagnetic radiation it is represented using mm, micrometers, meters etc.
Greek Letter Lambda (λ) is used for representing wavelength.
Frequency can be defined as number waves passing a fixed point in space. As electromagnetic radiations propagates at fixed velocity of light, higher the wavelength lesser the frequency. Wavelength is measured using hertz or cycles per second.
Based on frequency and wavelength electromagnetic energy is divided into several categories i.e., visible energy, infra red, microwaves etc.
Different Laws followed By Electromagnetic Radiation:
Wavelength and electromagnetic radiation can be related using the following law.
C = νλ
where, C = velocity of light (300,000,000 m) and
ν = frequency of electromagnetic radiation measured in cycles per second
λ = Wavelength measured in m
According to planks theory, All energy travels in the form of quanta. The magnitude of energy carried by a quantum is directly proportional to the frequency.
Planks theory can be shown with following equation.
E = hν
where, E = energy of a quantum
h = planks constant
ν = frequency
Planks can be related to wave theory with the below equation.
E = hc/λ
Stephan Boltzman Law:
This law explains black body radiation. All objects on earth surface having above absolute zero temperature emits radiation. The amount of radiation released from such bodies is a function of 4th power of their temperature.
Stephen Boltzman law is expressed using following equation:
M = σT4
Where, M = Radiant exitance,
T =temperature of object in Kelvin and
σ = Stephen boltzman constant = 5.6697 X 10 -8 Wm2K-4
Wiens Displacement Law:
The radiation from an object varies as per stephen boltzman law. With increase in temperature of an object, the radiation increases. However, the energy is distributed over a wavelength. The wave length at which maximum radiation is emitted by a black body object is given by Wiens displacement law.
According to wien’s displacement law, the maximum wavelength at which a blackbody releases its wavelength shifts to lesser wavelength with increase in temperature of the object.
The relation between maximum wavelength and black body temperature can be represented with the following equation.
λm = A/T
Where, λm = Wavelength
A = Constant =2898 µmK
T = Temperature of black body in Kelvins.