3 Interaction of EMR with surface and atmosphere

Dr. M P Punia

Objectives

Ø Student will acquire understanding of Earth’s surface.

Ø Student will acquire skill to analyze EMR interactions with different levels of atmosphere.

Ø Student will be equipped with knowledge to study Electromagnetic radiation interaction with surface      and atmosphere.

Outline

• Electromagnetic radiation principals and electromagnetic spectrum. Interaction of electromagnetic radiation with atmosphere.
• Interaction of electromagnetic radiation with Earth’s surface. Spectral response of materials.

    Introduction:

Electromagnetic (EM) radiation is a form of energy that is all around us in many forms, such as visible rays, radio waves, microwaves, X-rays and gamma rays. Our environment has always played a role with radiation, either originating naturally or manmade radiation. Radiation has always been associated with the nuclear energy and is easily misinterpreted. Heat radiations originating from Sun and thermal sources are the lifeline of planet.

The energy of radiation interacts with the environment through various atomic, molecular and nuclear mechanisms which can be usefully characterized by the amount of the energy involved in the process.

Energy in Radiation:

Energy incident on the earth’s surface from different source is absorbed, transmitted and reflected depending on the wavelength of radiation and characteristics of earth surface. Incident energy is reflected or re-emitted from the surface is recorded by sensors in satellite. These recorder signals are interpreted and analyzed to identify different features on surface.

Energy can be transferred from one matter to another matter by three basic mechanisms that are conduction, convection and radiation.

Conduction

It occurs when both the radiating (transferring) and the absorbing (receiving) bodies are in physical contact with each other. For example when a metal pan is heated by a hot plate, both energy transferring and the receiving bodies are in physical contact with each other. This process of energy transfer is known as conduction.

Convection

It is a process in which energy transferring from one place to another by physically moving of the bodies for example the heating of the air near the ground in the morning hours. The warmer air near the surface rises, setting up convectional currents in the stratosphere.

Radiation

Thermal radiation generated from the emission of electromagnetic waves. They carry energy away from the emitting source. Radiation is energy that comes from a source and travels through some materials or through space. All materials radiate thermal energy based on the temperature. Example, sun is the major source of radiating energy.

Electromagnetic Radiation

Electromagnetic radiation is the radiant energy released by electromagnetic process. Electromagnetic radiation consist of electromagnetic waves, which are synchronized oscillation of electric and magnetic field that travels with a speed of light (299,792,458 meters per second) through vacuum.

     Fig. 1 EMR wave

Source: http://www.livescience.com/38169-electromagnetism.html

The oscillations of two fields are perpendicular to each other and perpendicular to the direction of propagation of waves as shown in Fig. 1. Electromagnetic wave created whenever an electrical charged particle accelerated and these waves can

subsequently interact with any charged particle. EM waves carry energy, momentum and angular momentum away from their source particle and can impart those quantities to matter with which they interact. Wavelength (λ) is the mean distance between consecutive maximums or minimums and is measured in micrometers (µm) or nanometers (nm). Frequency (v) is the number of wavelength pass through a point per unit time. The relationship between the wavelength (λ) and frequency (v) of electromagnetic radiation is described by the following formula.

C= λv

v= C/λ

λ = C/v

The above formula shows frequency is inversely proportional to wavelength, means if wavelength will be maximum that time frequency will be minimum, shorter the wavelength higher will be the frequency. When electromagnetic radiation passes from one medium to another, the speed of light and wavelength changes while the frequency remains same.

Electromagnetic Spectrum

Electromagnetic waves is characterized by either frequency or wavelength, which determines the position in electromagnetic spectrum which consists of radio waves, microwaves ,infra-red waves, visible lights, Ultra violet rays, X-rays, Gamma rays as in Fig. 2.

    Fig. 2 Electromagnetic Spectrum

Sources: http://www.ces.fau.edu/nasa/module-2/radiation-sun.php

Radio Waves

Radio waves are the lowest range of electromagnetic spectrum with the frequency of 30 Gigahertz and wavelength greater than 10 millimeter. Radio waves are primarily used for communication including voice data. Objects in space such as planets and comets, giant clouds of gas and dust, and stars and galaxies, emit light at many different wavelengths. Some of the light they emit has very large wavelength- sometimes as long as mile. These long wavelengths are in the radio region of electromagnetic spectrum. Radio waves travel at speed of light, when passing through an object they are slowed according to that object’s permeability and permittivity.

Microwaves

Microwaves fall in range between radio and infra-red waves. They have a frequency of 3 GHz to 30 GHz and a wavelength of about 10 mm to 100 mm. They are used for high bandwidth communication, RADAR and microwave ovens. Radar wavelength and frequency used in active microwave remote sensing investigation as shown in Table1. Microwaves travel by line of sight, unlike lower frequency radio waves they do not diffract around hill, follow Earth surface as ground waves, or reflect from the ionosphere, so terrestrial microwave communication are limited by visual horizon to about 40miles.

Table 1

Radar Wavelength and Frequencies

    Infrared waves

Infrared falls between Microwave and Visible light range, they have a frequency range between 30 THz to 400 THz and wavelength of 100 µm to 740 nm. IR light is invisible to human eyes but the heat intensity can be felt. The basic applications of infrared get counted in the fields like military application and for civilian purposes. Military applications include target acquisition, surveillance, night vision, homing and tracking, non military uses include thermal efficiency analysis, environmental modeling, industrial facility inspection, remote temperature sensing, short ranged wireless communication, spectroscopy, weather forecasting etc. Relationships such as these suggest we can utilize radiometers placed at some distance from an object to measure its radiant temperature, which correlates well with the object’s true kinetic temperature; this is the basis of thermal infrared remote sensing temperature measurement (Fig. 3).

Fig. 3 Thermal image of an animal

Source: https://www.teachengineering.org/lessons/view/mis_sensor_lesson01

Visible Light

Our eyes can detect only tiny part of the electromagnetic spectrum, called visible light. Visible light are the wavelength which human eyes can see. Wavelength lies between the range of 380 nm to 740 nm and frequency about 400THz to 800 THz. White light is made up of whole range of colors mixed together. Each color has different wavelength, red has the longest wavelength where as violet has the shortest wavelength as shown in Fig. 4.

               Fig 4. Visual description of wavelength as it corresponds to the visible light spectrum

Ultraviolet

Ultra-violet light is in the range between visible light and X-rays. It has frequency of about 8*1014 to 3 *1016 Hz and wavelength of 380nm to 10nm. It has been used for various medical and industrial applications. UV wavelength of 320-400nm, called, UV-A, responsible for formation of Vitamin D by the skin, and on other hand causes sunburn and cataracts in eyes. UV-B falls within 290-320nm causes damage at the molecular level of fundamental building block of life-DNA, as it readily absorbs UV-B. The following diagram shows the various uses which EM spectrum provides in different bands Fig. 5.

                               Fig. 5 Uses Of different region of EM spectrum

          Source: https://www.britannica.com/science/ultraviolet-radiation

X-rays

An electromagnetic wave of high energy and very short wavelength, which is able to pass through much material opaque to light. Maximum of X rays has wavelength ranging from 0.01nm-10nm. X-rays are classified into two types: Soft X-rays and hard X-rays. Soft rays comprise the range of EM spectrum between UV and gamma rays, have the frequency of 3*1016 to about 1018 Hz. Hard Rays occupy the region same as gamma rays. X ray have the properties of penetrating various thicknesses of all solids. X rays hast vast applications in medical science field.

    Gamma Rays

Gamma rays have frequency greater than 1018 Hz and wavelength less than 100 Picometer. Gamma radiation causes damage to the tissues and can be used for curing cancer cells. Gamma rays are the most energetic form of light and are produced by hottest region of the universe. Natural source of gamma rays on earth are naturally occurring radionuclide’s, particularly potassium-40. Potassium-40 is a radioactive isotope of potassium, it can be found in soil, water and also in meat and banana.

Stefan-Boltzmann Law

The Stefan-Boltzmann law is named after two Austrian physicists, Joset Stefan and Ludwig Boltzmann. All objects above absolute zero (-273 c̊- or 0k) emits electromagnetic radiation. All objects above absolute zero (-273 c̊- or 0k) emit electromagnetic radiation sun is the initial source energy. Sun as a 5770-6000k black body, the critical construct that absorbs and radiates energy at the maximum possible rate per unit area at each wavelength (λ) for a given temperature. The total emitted radiation (j) from a black body measured is proportional to the fourth power of its absolute temperature (T) measured in Kelvin (k). This is known as the Stefan-Boltzmann law and is expensed as

j = σ T4, For a Black Body

σ  is Stefan-Boltzmann constant and has a value of

σ  = 5.6703 x 10-8 Wm-2 K-4

This signifies the amount of energy emitted by an object is the function of temperature of the body. The greater the temperature the greater the amount of radiating energy emitted by the object. It is summaries from explanation that the total emitted radiation from the book sun is for greater that emitted by the 300 k Earth.

Plank’s Radiation Law

Planck’s radiation law, a Mathematical relationship formulated in 1900 by German physicist Max Planck explained the spectral-energy distribution of radiation emitted by a blackbody (a hypothetical body that completely absorbs all radiant energy falling upon it, reaches some equilibrium temperature, and then reemits that energy as quickly as it absorbs it). Planck assumed that the source of radiation is atoms in a state of oscillation and that the vibrational energy of each oscillator may have any of a series of discrete values but never any value between. Planck further assumed that when an oscillator changes from a state of energy E1 to a state of lower energy E2, the discrete amount of energy E1 –E2, or quantum of radiation, is equal to the product of the frequency of the radiation, symbolized by the Greek letter v and a constant h, now called Planck’s constant, that he determined from blackbody radiation data;

i.e. E1- E2 = hv.

Planck’s law for the energy Eλ radiated per unit volume by a cavity of a blackbody in the wave length interval λ to λ + Δλ (Δλ denotes an increment of wavelength) can be written in terms of Planck’s constant (h) the speed of light (c), the Boltzmann constant (k), and the absolute temperature (T) :

8  ℎ                            1

λ =                  λ5           ∗            ℎ

(             λ)−1

The wavelength of the emitted radiation is inversely proportional to its frequency, or λ = c/v.

The value of Planck’s constant is found to be 6.62606957*10-34 joule.second, with a standard uncertainty of 0.00000029*10-34 joule. Second.

For a blackbody at temperatures up to several hundred degrees, the majority of the radiation is in the infrared radiation region of the electromagnetic spectrum. At higher temperatures, the total radiated energy increases, and the intensity peak of the emitted spectrum shifts to shorter wavelengths so that a significant portion is radiated as visible light.

Electromagnetic waves are one of the most common forms to undergo scattering.

    EMR interaction with atmosphere

Irrespective of the sources, all the radiation is detected by remote sensor. Particles and gases in the atmosphere can affect the incoming light and radiation. These effects are caused by mechanism of scattering and absorption.

A brief description of the Earth’s atmosphere is in order. The major components of dry air in the lower atmosphere are N2(78.1%) and O2(20.9%) with the remaining 1% composed of Ar, CO2, CH4, N2O, O3 and several other trace gases. Also, the lower atmosphere contains highly variable quantities of water vapour, usually making up 1 to 3% by volume. In addition to these molecules, there are solid and liquid particles such as many types of dust, salt from the oceans, spores, pollen, mist and clouds, raindrops, etc. The colloidal-sized particles (0.001-10 μm) are often referred to as aerosols.

The atmosphere has no definite thickness; its density and pressure decrease more or less exponentially with height above the surface. Traces of nitrogen and other molecules can be found out to 500 km but 99% of the atmosphere is below 30 km and half is below 6 km. The lower region, up to about 15 km, is called the troposphere. This is the region of strong convective mixing due to solar heating of the ground surface. This region contains a large fraction of the total atmospheric mass, particularly the trace gases, water vapor and particulate matter. Essentially all clouds and ‘weather’ are in the troposphere.

Above the troposphere is the stratosphere (from about 15 to 50 km) where the temperature rises again (due mainly to UVR absorption) to about -2ıC at 50 km. Above the stratosphere is the mesosphere (50- 85 km; temperature falling again to about -90ıC) and beyond 85 km is the thermosphere where the temperature rises again due to absorption of very high energy UVR (l < 100 nm) and X-rays (Fig 6).

                                     Fig. 6 Interaction of EMR with atmosphere

                                      Source: http://www.kau.edu.sa

Scattering (Definition)

Scattering is an optical phenomenon where by the radiant energy, while interacting with the atmosphere, deviate in all directions from its original path. Scattering theory is a framework for understanding of scattering of waves and particles. Scattering is the process by which “small particles suspended in a medium of a different index of refraction diffuse a portion of the incident radiation in all directions”. With scattering, there is no energy transformation, but a change in the spatial distribution of the energy. Scattering, along with absorption, causes attenuation problems with radar and other measuring devices.

Agents of Scattering

The incident solar radiation is reflected and scattered primarily by clouds (moisture and ice particles) particulate matter (dust, haze, and smog) and various gases. Atmospheric scattering disperses radiation in all directions. The important scattering agents are gaseous molecules, suspended particulates aerosols and clouds. Atmosphere modifies the frequency, intensity, the spectral distribution and the direction of the incident radiation passing through it.

Effects of Scattering

Scattering of EMR by the atmospheric continents leads to adverse effect in Remote Sensing .It reduces the image contrast, and it changes the reflectance characteristics (spectral signature) of ground objects as seen by the sensor.

Types of scattering:

There are three main types of scattering

Rayleigh scattering Mie Scattering

Nonselective scattering

Rayleigh scattering (molecular scattering)

Rayleigh scattering occurs when the effective diameter of the matter (usually air molecules) such as oxygen and nitrogen in the atmosphere are many times smaller (usually < 0.1) then the wavelength of the incident electromagnetic radiation. All scattering is accomplished through absorption and re-emission of radiation by atoms or molecules of air particles. It is impossible to predict the direction in which a specific atom or molecule will emit a photon, hence scattering. The energy required to excite an atom is associated with short-wavelength and high frequency radiation. The following graph shows the relation between wavelength and intensity of scattered light Fig. 7.

         Fig. 7 Plot of wavelength against intensity of scattered light

Source: Propagation, Dispersion and Scattering by Helen Amanda Fricker

The approximate amount of Rayleigh scattering in the atmosphere in optical wavelength (0.4-0.7) may be computed using the Rayleigh scattering cross-section

ζ

ζ=8  ³(n²-1)²\3N²λ

Where       n= refractive index

N= Number of air molecules per unit volume λ= Wavelength

The effect of Rayleigh scattering is inversely proportional to the wavelength that is shorter wavelengths are scattered more than the longer wavelengths. Most Rayleigh scattering by gas molecules takes places in atmosphere 2 to 8 km above the ground. Rayleigh scattering is responsible for blue appearance of the sky.

        Fig. 8 Rayleigh scattering

Source: Propagation, Dispersion and Scattering by Helen Amanda Fricker

The shorter violet and blue wavelengths are more efficiently scattered than the longer orange and red wavelengths. Rayleigh scattering is also responsible for red sunset since the atmosphere is a thin shell of gravitationally bound gas surrounding the solid Earth, sunlight must pass through a longer slant path of air at sunset than at noon (Fig. 9).

     Fig.9 Redness of sun during sunset

Source: Propagation, Dispersion and Scattering by Helen Amanda Fricker

Since the violet and blue wavelength are scattered even more during their now longer path through the air than when the sun is overhead what we see when we look toward the sunset is the reside the wavelengths of sunlight that are hardly scattered away all especially orange and blue (Fig. 8).

Mie Scattering

Mie scattering is also referred as non molecular or aerosol scattering. It generally takes place when the molecular size of the atmosphere is equal to the size of the wavelength or the incident energy. It normally takes place in the lower 4.5 km of the atmosphere. This scattering is caused by particles with radius between 0.1 and 10 µm such as dust, smoke an aerosols. The amount of scatter is greater than Rayleigh scatter and the wavelengths scattered are longer.

Higher amount of smoke and dust particles in the atmospheric column, the more violet and blue light will be scattered away and only the longer orange and red wavelength light will reach our eyes

Nonselective scattering

Nonselective scattering occurs when the atmospheric particle size is much greater than the incident radiation. This particles generally bound in the lowest part of atmosphere so nonselective scattering takes place in lower part of atmosphere. This is called nonselective scattering as wavelengths of light are scatter, not just blue, green or red. Thus the water droplets and ice crystals that make up clouds and fogbanks scatter all wavelength of visible light equally well, causing the cloud to appear white (Fig. 10).

Scattering can severely reduce the information content of remotely sensed data to the point that the imagery looses contrast and it is difficult to differentiate one object from another.

    Fig.10 Visibility distortions because of clouds and fogbanks

Source: Propagation, Dispersion and Scattering by Helen Amanda Fricker

Absorption

Apart from scattering, the molecules in the atmosphere absorb solar radiation. Most of the absorption occurs in broad bands in the IR region, i.e., l > 700 nm. This is due to vibrational absorption, primarily by water vapor molecules and to a lesser extent by CO2 molecules.

Since most of the water vapor is in the lower atmosphere, most of the IR absorption occurs in the lower 5 km of the troposphere. Although the energy is initially absorbed by the H2O and CO2 molecules, it is very rapidly (through molecular collisions) converted to random kinetic energy (heat energy) of all the molecules, resulting in warming of the lower atmosphere. In addition to vibrational IR absorption, there is also electronic absorption by atmospheric molecules, almost entirely by O2 and O3 (ozone) in the UV.

The small amount of very high energy UVR (l < 200 nm) and X-rays in solar radiation is absorbed by the equally small amount of O2 and N2 in the thermosphere and mesosphere (height > 50 km). Oxygen, and to a smaller extent N2, absorbs strongly in this spectral region and the photon energy is sufficient to both cleave, and ionize, the molecules as well as create excited electronic states. Most of the solar radiation of l > 200 nm penetrates into the stratosphere where there is further absorption by the increasing amount of O2.

Interaction of EMR with Earth’s Surface

Electromagnetic radiation that passes through the earth’s atmosphere without being absorbed or scattered reaches the earth’s surface to interact in different ways with different materials constituting the surface.

Radiation is able to penetrate the materials and pass through it is said to be transmitted. Most wavelength of visible light energy from sun is transmitted through the atmosphere, allowing it to come in contact with earth’s surface (Fig. 11).

          Fig.11 Interaction of EMR with Earth Surface Source: http://remote-sensing.net/concepts.html

There are three ways in which the total incident energy will interact with earth’s surface materials. These are Absorption Transmission, and Reflection

How much of the energy is absorbed, transmitted or reflected by a material will depend upon:

Wavelength of the energy

Material constituting the surface, and Condition of the feature.

Absorption:

Absorption of electromagnetic radiation is the way where energy of a photon is taken up by matter. Thus, the electromagnetic energy is transformed into internal energy of the absorber, for example thermal energy.

The reduction in intensity of a light wave propagating through a medium by absorption of a part of its photons is often called attenuation. Usually, the absorption of waves does not depend on their intensity (linear absorption), although in certain conditions (usually, in optics), the medium changes its transparency dependently on the intensity of waves going through, and saturable absorption (or nonlinear absorption) occurs.

    Measuring Absorption

The absorbance of the object is depended on the intensity of light absorbed by the matter. Precise measurement of the absorbance at many wavelengths allows the identification of substance via absorption spectroscopy, where a sample is illuminated from one side and the intensity of light exiting from all the direction is measured. Example, ultra-violet spectroscopy, infrared spectroscopy, X-ray absorption spectroscopy.

Transmission

Transmission is the process by which incident radiation passes through matter without measurable attenuation; the substance is thus transparent to the radiation. Transmission through material media of different densities (e.g., air to water) causes radiation to be refracted or deflected from a straight-line path with an accompanying change in its velocity and wavelength; frequency always remains constant.

Reflection

Reflection is a process in which energy is incident on the surface in such a way angle of incidence is equal to angle of reflection. When electromagnetic energy is incident on the surface, it may get reflected or scattered depending upon the roughness of the surface relative to the wavelength of the incident energy. If the roughness of the surface is less than the wavelength of the radiation or the ratio of roughness to wavelength is less than 1, the radiation is reflected. When the ratio is more than 1 or if the roughness is more than the wavelength, the radiation is scattered.

Reflection from surfaces occurs in two ways:

When the surface is smooth, we get a mirror-like or smooth reflection where all (or almost all) of the incident energy is reflected in one direction. This is called Specular Reflection and gives rise to images Fig 12(a).

When the surface is rough, the energy is reflected uniformly in almost all directions. This is called Diffuse Reflection and does not give rise to images Fig 12(b).

 Fig.12(a)  Specular Reflection                                               Fig.12(b) Diffuse Reflection

               Source: http://www.nrcan.gc.ca/node/14637

Most surface features of the earth lie somewhere between perfectly specular or perfectly diffuse reflectors. Whether a particular target reflects specular or diffusely or somewhere in between, depends on the surface roughness of the feature in comparison to the wavelength of the incoming radiation. If the wavelengths are much smaller than the surface variations or the particle sizes that make up the surface, diffuse reflection will dominate. For example, fine-grained sand would appear fairly smooth to long wavelength microwaves but would appear quite rough to the visible wavelengths.

Interaction of EMR with different material on Earth

Spectral Response of Materials:

By measuring the energy that is reflected (or emitted) by targets on the Earth’s surface over a variety of different wavelengths, we can build up a spectral response for that object. The spectral response of a material to different wavelengths of EMR can be represented graphically as a Spectral Reflectance Curve.

It may not be possible to distinguish between different materials if we were to compare their response at one wavelength. But by comparing the response patterns of these materials over a range of wavelengths (in other words, comparing their spectral reflectance curves), we may be able to distinguish between them. For example, water and vegetation may reflect somewhat similarly in the visible wavelengths but are almost always separable in the infrared (Fig. 14). Spectral response can be quite variable, even for the same target type, and can also vary with time (e.g. “green-ness” of leaves) and location.

     Fig 14. Spectral reflectance curve of vegetation, soil water

Source: http://www.seos-project.eu/modules/classification/classification-c00-p05.html

    Vegetation:

A chemical compound in leaves called chlorophyll strongly absorbs radiation in the red and blue wavelengths but reflects green wavelengths.

Leaves appear “greenest” to us in the summer, when chlorophyll content is at its maximum. In autumn, there is less chlorophyll in the leaves, so there is less absorption and proportionately more reflection of the red wavelengths, making the leaves appear red or yellow (yellow is a combination of red and green wavelengths) Fig 15.

              Fig.15 Internal structure of leaf and its reflectance characteristic Source: http://www.geol-amu.org/notes/mw4-2-4.htm

The internal structure of healthy leaves act as excellent diffuse reflectors of near-infrared wavelengths. If our eyes were sensitive to near-infrared, trees would appear extremely bright to us at these wavelengths. In fact, measuring and monitoring the near-IR reflectance is one way that scientists can determine how healthy (or unhealthy) vegetation may be.

Water:

Longer wavelength visible and near infrared radiation is absorbed more by water than shorter visible wavelengths. Thus water typically looks blue or blue-green due to stronger reflectance at these shorter wavelengths, and darker if viewed at red or near infrared wavelengths.

If there is suspended sediment present in the upper layers of the water body, then this will allow better reflectivity and a brighter appearance of the water. The apparent color of the water will show a slight shift towards longer wavelengths.

Suspended sediment (S) can be easily confused with shallow (but clear) water, since these two phenomena appear very similar. Chlorophyll in algae absorbs more of the blue wavelengths and reflects the green, making the water appear greener in colour when algae are present.

The topography of the water surface (rough, smooth, floating materials, etc.) can also lead to complications for water-related interpretation due to potential problems of specular reflection and other influences on colour and brightness.

We can see from these examples that, depending on the complex make-up of the target that is being looked at, and the wavelengths of radiation involved, we can observe very different responses to the mechanisms of absorption, transmission, and reflection.

Soil

Soil Surface is brown to human eyes, as it is a combination of green and red EMR. A very little amount of energy transmitted through soil and most of it is either absorbed or reflected. In Soil surface, level of reflectance gradually increases with the increase of wavelength in visible and IR regions. Fig. 16.

Fig.16 Spectral response curve of different type of soil Source: http://www.geol-amu.org/notes/mw4-2-4.htm

Presence of soil moisture reduces the surface reflectance at all visible wavelength. Reflectance at near infrared wavelength is also negatively to soil moisture. An increase in soil moisture will result in rapid decrease in reflectance due to water.

A clay soil tends to have a strong structure which leads to a rough surface on ploughing, causing shadows and lower reflectance. Sandy soil exhibits weak structure which leads to fairly smooth surface.

Summary

Energy can be transferred from one matter to another matter by three basic mechanisms that are conduction, convection and radiation. Radiation is energy that comes from a source and travels through some materials or through space. All materials radiate thermal energy based on the temperature, hotter the object more radiation will occur. Electromagnetic radiation consists of electromagnetic waves, which are synchronized oscillation of electric and magnetic field that travels with a speed of light through vacuum. Electromagnetic Radiation interacts with particles and gases in the atmosphere by the mechanism of scattering and absorption.

Scattering is an optical phenomenon where by the radiant energy, while interacting with the atmosphere, deviate in all directions from its original path. Scattering can be divided into Rayleigh scattering, Mie Scattering & Nonselective scattering. Rayleigh scattering is responsible for blue appearance of the sky & for red sunset. Due to the absorption of radiation through molecules in atmosphere, it results in warming of the lower atmosphere.

Electromagnetic Radiation is able to penetrate the materials and pass through it is said to be transmitted. Most wavelength of visible light energy from sun is transmitted through the atmosphere, allowing it to come in contact with Earth’s surface. Absorption (A) occurs when radiation (energy) is absorbed into the target while, Reflection (R) occurs when radiation “bounces” off the target and is redirected.