8 Electromagnetic Radiations
Dr. Puneeta Pandey
1. Learning Objectives
The purpose of this chapter is:
- To understand the general properties of electromagnetic radiations
- To emphasize the idea how electromagnetic radiations help in remote sensing
- To explain the impact of the atmosphere in absorption and scattering of electromagnetic energy
2. Electromagnetic Spectrum
Electromagnetic radiations are transverse waves travelling with the speed of light in vacuum (3×108m/s). For an electromagnetic radiation, the electric (E) and magnetic (M) fields vary sinusoidally, both being perpendicular to each other and to the direction of progression of wave (Figure 1).
Figure 1: Electromagnetic wave
As shown in Figure 1, electromagnetic radiations travel at the velocity of light, c. The distance from one wave peak to the next is wavelength (λ), and the number of peaks passing a fixed point in space per unit time is the wave frequency (v).
From basic physics, waves obey the general equation
= vλ—————-(1.1)
Since c is essentially a constant (3×108 m/sec), both frequency and wavelength can be used to characterize a wave, and have an inverse relation with each other. In remote sensing, it is most common to categorize electromagnetic radiations are characterized by their wavelength location in the electromagnetic spectrum. The commonly used unit to measure wavelength is micrometer (µm); which equals 1×10-6m.
Generally, when we talk about electromagnetic radiations, we commonly refer to them as light or simply radiations. However, this light is the visible radiation, which our eyes can perceive; and comprise of a small portion of the electromagnetic spectrum. The other form of radiations include gamma radiations, X-rays, ultra-violet, infra-red and microwave radiations besides TV and radio waves.
Further, it is to be understood that these radiations do not have a clear cut demarcating line among them; and follow a continuum from gamma rays to TV and radio waves in terms of wavelength, frequency and energy. This continuum is known as electromagnetic spectrum.
Figure 2: Electromagnetic Spectrum
These radiations are briefly discussed below:
2.1. Gamma rays
Gamma rays are high frequency and high energy waves, with a wavelength shorter than 0.1 Angstrom. Gamma rays are used in ‘Radiotherapy’ to kill cancer cells, to sterilize medical instruments and kill harmful micro-organisms in industries.
2.2. X-rays
X-rays are also very high frequency waves, with a wavelength range of 0.01 to 10 nanometers (nm). They are useful in medicine for medical diagnosis and in industries for correcting flaws of welded metal joints.
2.3. Ultra-Violet radiations
Ultraviolet (UV) energy adjoins the blue end of the visible portion of the spectrum with a wavelength range of 10 nm to 400 nm. It was first discovered by a German scientist Johann Wilhelm Ritter in 1801 and lies between X-rays and visible light in electromagnetic spectrum. Ultraviolet region is sub-divided into near Ultra-violet (UV-A : 0.32-0.40 µm ); far Ultra-violet (UV-B : 0.28- 0.32 µm ) and extreme Ultra-violet (less than 0.28 µm ). UV light is used in sterilizing microbial contamination in medical domain, hardening dental filling, in biochemistry and pharmaceutical industries. Further, since UV light is easily scattered by the atmosphere, it is not used for remote sensing purposes.
2.4. Visible radiations
Visible light lies in the range of 0.4-0.7µm corresponding to spectral sensitivity of human eye. The color blue is ascribed to range of 0.4 to 0.5 µm, green to 0.5 to 0.6 µm and red to 0.6 to 0.7 µm. White light is made up of various wavelengths in visible regions ranging from violet to red, i.e., the colours of the rainbow. Visible light finds applications in photography.
2.5. Infrared (IR) radiations
Adjoining the red end of visible region are three different categories of infrared (IR) waves: near IR (from 0.7 to 1.3 µm), mid IR (from 1.3 to 3 µm) and thermal IR (beyond 3 to 14 µm). Infra-red waves are released as heat by longer-wavelength infrared waves, and are used for remote controls for Television in shorter-wavelength infrared rays; besides healing sports injuries by physiotherapists and for weather forecasting by infrared satellite data.
2.6. Microwaves
Microwave ranges from wavelengths of 1mm to 1m. They are used in cooking, mobile phones, radar, aircraft and weather forecasting. Microwaves are useful in communication because they can penetrate clouds, smoke, and light rain.
2.7. Radiowaves
These waves have the longest wavelengths and the lowest frequencies among all the radiations of electromagnetic spectrum. They are divided into the following, based on the wavelength range:
2.7.1 Long Wave: These have wavelength of about 1-2 km.
2.7.2 Medium Wave: These waves have wavelength of approximately 100m
2.7.3 Very High Frequency (VHF) Waves: These waves are characterized by wavelengths of about 2m, and find applications in communication.
2.7.4 Ultra High Frequency (UHF) waves: These waves have wavelengths of less than a meter, and are used mainly for communications such as Police radio, military aircraft and television transmissions.
Out of these radiations of electromagnetic spectrum, the remote sensing systems operate in one or several of the visible, Infrared or microwave portions of the spectrum. Gamma rays, X-rays and ultraviolet rays cannot be used in remote sensing due to being high energy waves; while TV and radio waves have very low energy; hence, it would be difficult to record the signals received by these waves.
3. Principles of Radiation
Electromagnetic radiations exhibit both wave and particle nature. The following laws define the properties of electromagnetic radiations that have been dealt below:
3.1 Planck’s Law
This law is based on particle nature of radiations and states that electromagnetic radiation is composed of many discrete units called photons or quanta. The energy of a quantum is given as
Q=hv————–(1.2)
where,
Q= energy of a quantum, joules (J)
h= Planck’s constant, 6.626× 10-34 J sec
v= frequency
Both the wave and particle nature can be related to each other by solving Eq. 1.1 for v and substituting into Eq. 1.2 to obtain
Q=h/λ—————(1.3)
Thus, it can be derived from Eq. 1.3 that the energy of radiation is inversely proportion to its wavelength. The longer the wavelength involved, the lower its energy content. This has important implications in remote sensing since long wavelength radiations such as microwave emission from terrain features are difficult to sense than radiation of shorter wavelengths such as emitted thermal IR energy. This implies that to obtain a detectable energy signal, a sensor must view large areas of the Earth at any given time.
3.2 Stefan-Boltzmann law
All matter at temperatures above absolute zero (0K or -273°C) continuously emits the electromagnetic radiation. The amount of energy radiated by an object is a function of its surface temperature. This property is expressed by the Stefan-Boltzmann law, which states that
M=σT4——————(1.4)
where,
M= total radiant exitance from the surface of a material, watts (W) m-2
σ= Stefan-Boltzmann constant, 5.6697×10-8 W m-2 K-4
T= absolute temperature (K) of the emitting material
Thus, the total energy emitted from an object increases rapidly with increase in temperature. This law generally holds for an energy source that behaves as a blackbody (a hypothetical, ideal radiator that totally absorbs and re-emits all energy incident upon it).
3.3 Wien’s Displacement Law
Spectral distribution of the emitted energy varies with temperature. Figure 3illustrates graphically the mathematical expression of Stefan-Boltzmann law, and shows energy distribution curves for blackbodies at temperature ranging from 200 to 6000K. The units on the ordinate scale (W m-2 µm-1) express the radiant power coming from a blackbody per 1 µm spectrum interval. Therefore, the area under these curves equals the total radiant exitance M, and the curves: the higher the temperature of the radiator the greater the total amount of radiation it emits. The curves also show that there is a shift toward shorter wavelengths in the peak of a blackbody radiation distribution as temperature increase. The dominant wavelength, or wavelength at which a blackbody radiation curve reaches maximum, is related to its temperature by Wien’s displacement law:
λm =A/T
Where,
λm = wavelenght of maximum spectral radiant exitance,
µm A= 2898 µm K
T= temperature, K
Thus for a blackbody, the wavelength at which the maximum spectral radiant exitance occurs varies inversely with the blackbody’s absolute temperature.
Figure 3: Spectral distribution of energy radiated from blackbodies of various temperatures (Lillesand et. al., 2008; 6th Edition)
It is to be noted that the Earth’s ambient temperature is about 300K (27°C); therefore, the maximum spectral radiant exitance from earth features occurs at a wavelength of about 9.7 µm, which falls in thermal infrared region of spectrum. Comparatively, the spectral radiant exitancefrom sun occurs at about 0.5 µm, falling in visible portion of the spectrum. This is why sunlight can be sensed by human eye or a photographic system; while the radiation from the earth can be perceived as heat or by a non-photographic sensing system.
4. Energy Interactions in the Atmosphere
All radiations whether from active or passive source; which is detected by remote sensors, passes through some distance (or path length) of the atmosphere. During this travel, the radiations interact with the atmosphere and undergo the process of reflection, absorption or scattering. In remote sensing, the mechanism of atmospheric scattering and absorption plays an important role.
4.1 Scattering
Atmospheric scattering is the unpredictable diffusion of radiation by particles in the atmosphere.
Figure 4: Mechanism of scattering (http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/opt/mch/sct.rxml)
Based on the size of the particle and the wavelength, there are three types of scattering (Figure 5):
• Rayleigh scattering
• Mie scattering
• Nonselective scattering
4.1.1 Rayleigh scattering
It occurs when atmospheric particles’ diameters are much smaller than the wavelength of the radiation (d<<λ). It is common high in the atmosphere. Scattering intensity is proportional toλ-4. Radiation with shorter wavelength is easier to be scattered by this mechanism; therefore, there is much stronger tendency for short wavelengths to be scattered by mechanism than long wavelengths. It is primarily caused due to oxygen and nitrogen molecules. An example of Rayleigh scatter is the blue colour of sky. In the absence of scatter, the sky would appear black. But, as sunlight interacts with the Earth’s atmosphere, it scatters the shorter (blue) wavelengths more dominantly than the other visible wavelengths. Consequently, we see a blue sky. At sunrise and sunset, however, the sun’s rays travel through a longer atmospheric path length than during midday. With the longer path, the scatter of short wavelengths is so complete that we see only the less scattered, longer wavelengths of orange and red. Rayleigh scatter is one of the primary causes of haze in imagery. Visually, haze diminishes the crispness or contrast of the image.
Figure 5: Types of scattering
4.1.2Mie scattering
Mie scattering occurs when the diameter of atmospheric particles equals the wavelength of the energy being sensed. This is caused by water vapor and dust and influences longer wavelengths compared to Rayleigh scatter. Mie scatter is significant in slightly overcast ones.
4.1.3 Nonselective Scattering
Non-selective scattering occurs when the diameters of the particles are much larger than the wavelengths of the energy being sensed. Such scattering is caused by water droplets, ice crystals, volcanic ash, smog. These particles generally have a diameter in the range 5 to 100 µm and scatter all visible and near to mid-IR wavelengths about equally. Consequently, this scattering is non-selective with respect to the wavelength. In the visible wavelengths, equal quantities of blue, green and red light are scattered, hence fog and clouds appear white.
4.2 Absorption
Atmospheric absorption results in the effective loss of energy to atmospheric constituents. This is normally caused by water vapor, carbon dioxide and ozone. The wavelength ranges in which the atmosphere is particularly transmissive of energy are referred to as atmospheric windows.
Figure 6 shows the interrelationship between the energy sources and atmospheric absorption characteristics. Figure 6(a) shows the spectral distribution of the energy emitted by the sun and (b) the earth features. In figure 6(b), spectral regions in which the atmosphere blocks energy are shaded. Remote sensing data acquisition is limited to the non-blocked spectral regions, which are known as atmospheric windows. Figure 6(c) describes the remote sensing systems corresponding to the wavelengths of electromagnetic spectrum. For example, the spectral sensitivity of the eye coincides with the visible region; thermal scanners correspond to 3 to 5 µm and 8 to 14 µm of wavelength. Multispectral scanners sense simultaneously through multiple wavelength ranges from visible to the thermal spectral region. Passive microwave systems operate through a window in the region 1 mm to 1 m.
Figure 6: Spectral characteristics of (a) energy sources, (b) atmospheric transmittance, and (c) common remote sensing systems.
In remote sensing, it becomes essential to consider the following criterion:
(A) Spectral sensitivity of the sensors available
(B) Presence or absence of the atmospheric windows in the spectral range in which one wishes to sense
(C) Source, magnitude and spectral composition of the energy available in these ranges.
- Summary
To conclude, an electromagnetic radiation can be described by its amplitude, wavelength, frequency, and time period. These electromagnetic radiations are transverse waves having both particle and wave nature. By the Planck’s law, we know that the frequency of a light wave is proportional to its energy. This has implications for remote sensing studies since higher energy waves such as X-rays and UV light get scattered sooner; while low energy waves such as Microwave and radio waves would require much higher areal coverage to obtain a detectable energy signal. Further, inn this module, we have also studies the various principles pertaining to electromagnetic radiations; besides studying the mechanism of scattering and absorption-two essential factors for design of remote sensors.