Concept of Electromagnetic Spectrum
Modern physics views EMR as having dual nature, enabling it to be independently described as a wave or a particle.
The Wave Model shows EMR as being carried by a series of continuous waves that are equally and repetitively spaced in time (harmonic waves)
Wave pattern is in the form of 2 fluctuating fields - one electric and the other magnetic. Each has a sinusoidal shape because their plots resemble sine curves.
The paired fields are perpendicular to each other, and both are perpendicular to direction of wave propagation (transverse waves)
Wave nature of EMR is characterized by:
Wavelength (lambda) - linear distance between 2 successive wave crests or troughs
or f) - # of wave crests or troughs (cycles) that pass a fixed point per
Wavelength and frequency are related to the velocity of an electromagnetic wave (speed of light) -
speed of light (c) = frequency (f) X wavelength (lambda) ,,,, (1)
- frequency and wavelength are directly
proportional to velocity which is essentially a constant
The Particle Model emphasizes behavior of EMR as if EMR were composed of a collection of discrete, particle-like objects called quanta or photons, in which electromagnetic energy is transferred at the speed of light.
Energy of a quantum is given as:
Q = h f = (h c) / lambda ..... (2)
Q - energy of quantum [Joules - J]
Relate wave model and quantum model of emr
(Equation 1 and 2)
This equation shows that the shorter the wavelength, the higher the energy.
For this reason, shorter wavelengths are easier to sense than very long ones such as passive terrestrial microwave emissions
EMS represents the continuum of electromagnetic energy from extremely short wavelengths (cosmic and gamma rays) to extremely long wavelengths (microwaves). Spectrum is arbitrarily segmented into major divisions. There are no natural breaks in the ems. These separations are made by us for our convenience.
ULTRA VIOLET - 3 nanometers - .4 micrometers
VISIBLE - small portion of the EMS that humans are sensitive to
BLUE (.4-.5 micrometers)
GREEN (.5-.6 micrometers)
RED (.6-.73 micrometers)
- .72 - 15 micrometers
NEAR INFRARED - reflected, can be recorded on film emulsions. (0.7 - 1.3 micrometers)
MID INFRARED - reflected, can be detected using electro-optical sensors. (1.3 - 3.0 micrometers)
THERMAL INFRARED - emitted, can only be detected using electro-optical sensors. (3.0 - 5.0 and 8 - 14 micrometers)
MICROWAVE - Radar sensors, wavelengths range from 1mm to 1m
Remote sensing is concerned with the measurement of EMR returned by the Earth's natural and man-made features that first receive energy from the sun or an artificial source such as a radar transmitter.
Different objects return different types and amounts of EMR.
Objective of remote sensing is to detect these differences with the appropriate instruments.
Differences make it possible to identify and assess a broad range of surface features and their conditions
This website is hosted by
Department of Geology
Aligarh Muslim University, Aligarh - 202 002 (India)