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

         frequency

Wavelength (lambda) - linear distance between 2 successive wave crests or troughs

Frequency  (v or f) - # of wave crests or troughs (cycles) that pass a fixed point per second
 

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
- electromagnetic energy travels at the speed of light 2.99983x108  (3x108 ) ms-1  (186,000 miles s-1)
- wavelength and frequency have an inverse relationship

 

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]
h - Plank's constant [6.26x10-34 J s]

  • There is a direct relationship between frequency and energy (energy of a photon varies directly with frequency)

  • There is an inverse relationship between wavelength and energy (energy of a photon varies inversely with wavelength)

Relate wave model and quantum model of emr (Equation 1 and 2)
 

1. solving for f

yielding 

2. substituting intoQ=hf

 

yielding 

 

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

Causes fluorescence and is good in some geological and vegetation applications.

Big sagebrush (artemisia spp.) fluorecess under ultra-violet light. Some flowers also fluoresce under UV light allowing insects to locate nectar reservoirs.

Not much is done with UV for remote sensing since these shorter wavelengths are easily scattered by the atmosphere making spaceborne and some airborne sensors impractical.


VISIBLE - small portion of the EMS  that humans are sensitive to

BLUE (.4-.5 micrometers)

GREEN (.5-.6 micrometers)

RED (.6-.73 micrometers)


INFRARED SPECTRUM - .72 - 15 micrometers
- There are three logical zones in the IR spectrum:

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


Summary

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

S. Farooq

Department of Geology

Aligarh Muslim University, Aligarh - 202 002 (India)

Phone: 91-571-2721150

email: farooq.amu@gmail.com