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 in radiation, either originating naturally or manmade radiation. Radiation has always been associated with nuclear energy and is easily misinterpreted. Heat radiations originating from Sun and thermal sources are the lifeline of the 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 energy involved in the process.
After electromagnetic radiation has been created by the Sun, the part of it that has found its way through the vacuum of space to the top of the Earth’s atmosphere must pass through the atmosphere, be reflected by the Earth’s surface, pass through the atmosphere again on its way back to space, and then arrive at the sensor in order to be recorded. While nothing happens to the radiation field as it passes through empty space, several things happen as it interacts with the Earth’s atmosphere and surface. Due to these interactions, the measured radiation ends up containing information about the Earth’s environment, so it is essential to take a closer look at precisely what happens in these interactions and how it affects the radiation field.
Interactions with atmosphere
The interaction between electromagnetic radiation and the Earth’s atmosphere can be considered to have three components:
Scattering
One of the two remaining processes that influence electromagnetic radiation as it passes through the atmosphere is scattering. Scattering happens when a photon interacts with something in the atmosphere that causes it to change direction. Depending on the size of the object that the photon interacts with, two distinct types of scattering are recognized. Rayleigh scattering happens when the object is much smaller than the wavelength of the radiation. In the case of sunlight and the Earth’s atmosphere, this means that Rayleigh scattering is caused by atmospheric gasses like N2, O2, CO2, etc. Mie scattering happens when the object is similar in size to the wavelength of the radiation, which means that it is caused by aerosols like smoke and dust particles. Additional scattering can happen if radiation interacts with particles larger in size than its wavelengths, like water droplets or sand particles.
Scattering is a physical phenomenon in which energy, such as light or sound, is redirected or dispersed in different directions as it interacts with matter. In the case of light, scattering occurs when photons interact with particles in the atmosphere, such as air molecules, water droplets, and dust particles. The scattering of light is responsible for many natural phenomena, including the blue color of the sky, the red color of sunsets, and the white appearance of clouds.
There are three main types of scattering:
1. Rayleigh Scattering
When particles (pure gas molecules) are very small (10-4 ƛm) compared to the wavelength of radiation (0.4-0.76 µm). This scattering is the dominant mechanism in the upper atmosphere that causes the scattering of shorter wavelengths than longer wavelengths, thus giving blue color to the sky.
This is caused by particles smaller than the wavelength and is maximum for small wavelength.
A fact that has great importance for remote sensing of the Earth is that the magnitude of Rayleigh scattering is inversely related to the 4th power of the wavelength of the radiation. In other words, radiation with shorter wavelengths is scattered much more by Rayleigh scattering than the radiation at longer wavelengths.
In the visible wavelengths, this means that blue light is scattered more than green light, which in turn is scattered more than red light. This is the process that makes the Earth’s oceans look blue when viewed from space.
Rayleigh scattering is a type of scattering that occurs when light interacts with particles that are much smaller than the wavelength of the light. This type of scattering is named after the British scientist Lord Rayleigh, who first described it in the late 19th century.
The intensity of Rayleigh scattering is proportional to the fourth power of the inverse wavelength of the light. This means that shorter wavelength light, such as blue light, is scattered much more than longer wavelength light, such as red light. This is why the sky appears blue during the day, as the shorter wavelength blue light is scattered more by the atmosphere than the longer wavelength red light.
Rayleigh scattering is the dominant type of scattering in the Earth’s atmosphere. It is responsible for the blue color of the sky, as well as the reddish-orange color of sunsets and sunrises. During sunrise or sunset, the light has to travel through a greater amount of the atmosphere before reaching the observer, and much of the shorter wavelength blue light is scattered away, leaving only the longer wavelength red and orange light to be seen.
Examples of Raleigh Scattering
Rayleigh scattering is a common phenomenon that can be observed in many different settings. Here are some examples of Rayleigh scattering:
- Blue color of the sky: The blue color of the sky is due to Rayleigh scattering of sunlight by the molecules in the Earth’s atmosphere. Shorter wavelength blue light is scattered much more than longer wavelength red light, so the sky appears blue during the day.
- Red and orange colors of sunsets and sunrises: During sunrise or sunset, the light has to travel through a greater amount of the atmosphere before reaching the observer. As a result, much of the shorter wavelength blue light is scattered away, leaving only the longer wavelength red and orange light to be seen.
- Polarization of light: Rayleigh scattering can also cause light to become polarized, meaning that the electric field of the light is oriented in a specific direction. This can be observed in the blue sky, where the scattered light is polarized perpendicular to the direction of the sun.
- Color of water: Rayleigh scattering can also affect the color of water. In clear water, blue light is scattered more than other colors, making the water appear blue. In water with suspended particles, such as algae or sediment, the color of the water can be affected by other types of scattering.
- Astronomy: Rayleigh scattering is an important phenomenon in astronomy. It can be used to study the composition of the atmospheres of planets and other celestial bodies, as well as to determine the distances to stars and galaxies.
2. Mie Scattering
When particles (smoke, blaze) are just about the same size (0.1 10µm) as the wavelength of radiation (ƛ0-ƛ-4 ).
This is caused by the particles equal to the wavelength.
Mie scattering, because its strength and wavelength dependence depend on the type and density of the particulates that cause it to happen, varies substantially through time and space. As a result, it is one of the most important causes of uncertainty in remote sensing, especially when using satellite data to study dark parts of the Earth’s surface from which the amount of reflected radiation is small relative to the total signal from atmospheric scattering.
For the same reason it is hard to generalize its importance, but broadly speaking the strength of Mie scattering exceeds that of Rayleigh scattering, and while it still diminishes with increasing wavelength its influence extends further into the infrared spectrum. Because Mie scattering is caused by atmospheric particulates, it is often dramatically increased during dust storms, forest fires, or other events that caused the atmospheric aerosol load to increase.
Mie scattering is a type of scattering that occurs when light interacts with particles that are about the same size as the wavelength of the light. This type of scattering is named after the German physicist Gustav Mie, who first described it in the early 20th century.
Unlike Rayleigh scattering, which depends on the size of the scattering particles being much smaller than the wavelength of the light, Mie scattering occurs when the size of the scattering particles is similar to or larger than the wavelength of the light. Mie scattering is responsible for the white appearance of clouds, the haze that can be seen in the air, and the bluish tint of some iridescent objects, such as peacock feathers.
Mie scattering depends on the size and refractive index of the scattering particles, as well as the wavelength of the incident light. The intensity of Mie scattering is generally not strongly dependent on the wavelength of the light, so particles that scatter light through Mie scattering appear white or gray.
Mie scattering is an important phenomenon in many fields, including atmospheric science, materials science, and biological imaging. It is used to study the properties of particles in the atmosphere, such as aerosols and pollutants, as well as to understand the behavior of light in biological tissues, such as the eye.
For example
Examples of Mie Scattering
Mie scattering is a common phenomenon that can be observed in many different settings. Here are some examples of Mie scattering:
- White appearance of clouds: Mie scattering is responsible for the white appearance of clouds. Clouds are made up of water droplets or ice crystals that are about the same size as the wavelength of visible light, so they scatter all colors of light equally and appear white.
- Haze in the air: Mie scattering can also cause a haze to appear in the air. This can occur when there are a large number of particles in the air, such as dust, pollen, or smoke. The particles scatter light through Mie scattering, causing the air to appear hazy.
- Bluish tint of iridescent objects: Mie scattering can also cause iridescent objects, such as peacock feathers, to appear blue. This occurs when the size of the scattering particles is similar to the wavelength of the incident light and the refractive index of the particles varies with wavelength. This causes different colors of light to be scattered in different directions, resulting in a bluish tint.
- Colored appearance of some liquids: Mie scattering can also affect the color of some liquids, such as milk and paint. This occurs when the size of the scattering particles is similar to the wavelength of the incident light and the refractive index of the particles is different from that of the surrounding medium. This causes different colors of light to be scattered in different directions, resulting in a colored appearance.
- Biological imaging: Mie scattering is also used in biological imaging techniques, such as confocal microscopy and optical coherence tomography. These techniques use Mie scattering to image tissues and cells in the body, allowing researchers and medical professionals to diagnose and treat diseases.
3. Nonselective Scattering
When particles are much larger (>10 µm) than the wavelength of radiation (ƛ0 ).
Nonselective scattering can happen if radiation interacts with particles larger in size than its wavelengths, like water droplets or sand particles.
Nonselective scattering is a type of scattering where all wavelengths of light are scattered with equal probability, regardless of the size or composition of the scattering particles. Unlike Rayleigh and Mie scattering, which are selective and depend on the size and composition of the scattering particles, nonselective scattering is independent of the particle properties and is instead caused by irregularities or roughness on the surface of the material.
Nonselective scattering can occur in a wide range of materials, including metals, plastics, and ceramics. It is responsible for the diffuse reflection of light from these materials, which gives them their matte appearance. Nonselective scattering can also be used to create diffuse reflectors for lighting applications, such as in photography and cinematography.
One important example of nonselective scattering is Lambertian scattering, which is a type of nonselective scattering where the intensity of the scattered light is independent of the angle of observation. Lambertian scattering is an idealized type of scattering that is used to model the behavior of diffuse reflectors in many applications.
Examples of Nonselective Scattering
Nonselective scattering is a common phenomenon that can be observed in many different materials and settings. Here are some examples of nonselective scattering:
- Matte surfaces: Nonselective scattering is responsible for the matte appearance of many surfaces, such as paper, paint, and plastics. The irregularities on the surface of these materials scatter light in all directions, giving the surface a uniform appearance.
- Diffuse reflectors: Nonselective scattering is also used to create diffuse reflectors for lighting applications, such as in photography and cinematography. Materials such as white paint, matte paper, and some types of plastics are commonly used as diffuse reflectors because they scatter light in all directions, creating a uniform and evenly lit surface.
- Rough surfaces: Nonselective scattering can occur on rough surfaces, such as those of rocks or soil. The irregularities on the surface of these materials scatter light in all directions, resulting in a non-glossy appearance.
- Snow: Snow is a natural example of nonselective scattering. The surface of snow is made up of irregular ice crystals, which scatter light in all directions and give snow its white appearance.
- Metallic surfaces: Nonselective scattering can also occur on metallic surfaces. Metallic surfaces have microscopic irregularities on their surface, which scatter light in all directions and give them their characteristic matte appearance. This is used in many applications, such as in architectural metalwork and automotive finishes.
Some Other Information Interactions with atmosphere
The atmosphere interacts with the Earth’s surface and the energy that comes from the sun. These interactions play a crucial role in shaping the climate and weather patterns on Earth. Here are some of the ways the atmosphere interacts with its surroundings:
- Absorption and reflection of solar radiation: The atmosphere absorbs some of the sun’s energy and reflects the rest back into space. This helps regulate the temperature of the Earth’s surface.
- Greenhouse effect: Certain gases in the atmosphere, such as carbon dioxide and water vapor, trap some of the sun’s energy and prevent it from escaping back into space. This creates a warming effect known as the greenhouse effect.
- Convection: As the Earth’s surface absorbs the sun’s energy, the air near the surface heats up and rises. This sets up a cycle of convection, in which warm air rises and cooler air sinks. This creates winds and weather patterns.
- Precipitation: Water vapor in the atmosphere condenses into clouds, and when the clouds become saturated, they release their moisture as precipitation, such as rain or snow.
- Atmospheric pressure: The atmosphere exerts pressure on the Earth’s surface, which helps regulate weather patterns and create wind. Differences in atmospheric pressure can cause wind to blow from high-pressure areas to low-pressure areas.
These are just a few examples of the complex interactions between the atmosphere and the Earth’s surface. These interactions play a critical role in maintaining the delicate balance of the Earth’s climate.
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. The most wavelength of visible light energy from the sun is transmitted through the atmosphere, allowing it to come in contact with the earth’s surface.
There are three ways in which the total incident energy will interact with Earth’s surface materials. These are
- Absorption
- Transmission
- 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
- Condition of the feature
Absorption
occurs when radiation (energy) is absorbed into the target. (A)
Absorption of electromagnetic radiation is the way where the energy of a photon is taken up by matter. Thus, electromagnetic energy is transformed into internal energy of the absorber, for example, thermal energy.
The reduction in the 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.
Absorption is the process by which electromagnetic radiation (EMR) is absorbed by a material. When EMR interacts with a material, the radiation can be absorbed by the material’s atoms and molecules, causing them to vibrate or move. The amount of absorption depends on the wavelength of the radiation and the properties of the material.
Different materials have different absorption properties, and some materials absorb radiation at specific wavelengths or bands. For example, green vegetation absorbs more radiation in the red and blue parts of the visible spectrum and reflects more in the green part, which is why plants appear green. Infrared radiation is absorbed by many materials, including water, soil, and vegetation.
Absorption can have important effects on the energy balance of the Earth’s atmosphere and surface. When radiation is absorbed by the Earth’s surface or atmosphere, it can be converted into heat energy, which can warm the surface or the air. This process is called radiative forcing and can have significant impacts on climate and weather patterns.
Transmission
Occurs when radiation passes through a target. (T)
When electromagnetic radiation is incident on Earth’s surface, part of the energy gets scattered from the surface (which is known as surface scattering)and a part of the energy gets transmitted into the medium. In homogeneous materials, the radiation is simply transmitted but in inhomogeneous materials, the transmitted radiation gets further scattered
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.
Transmission is the process by which electromagnetic radiation (EMR) passes through a material without being absorbed or reflected. The amount of transmission depends on the wavelength of the radiation and the properties of the material. Different materials have different transmission properties, and some materials are transparent to certain wavelengths or bands of radiation while others are not.
For example, visible light can pass through air and water, making them transparent to the human eye. However, some materials like metals are opaque to visible light and absorb or reflect it instead. Infrared radiation can also pass through air and some materials like glass and plastic, but is absorbed by others such as metal and water.
The transmission of EMR has important applications in many fields, such as optics, telecommunications, and remote sensing. For example, the transmission of visible light through optical fibers is used in telecommunications to transmit information over long distances. The transmission of microwave and radio waves through the atmosphere is used for communication and weather forecasting, as well as in radar systems.
Reflection
Occurs when radiation “bounces” off the target and is redirected. (R)
Reflection is a process in which energy is incident on the surface in such a way angle of incidence is equal to the 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 is the process by which electromagnetic radiation (EMR) is reflected off a surface without being absorbed or transmitted through it. The angle at which the radiation is reflected is determined by the angle at which it strikes the surface and the surface’s properties.
The reflection of EMR plays an important role in many natural and artificial processes. For example, the reflection of visible light off objects determines how they appear to the human eye. When light strikes a smooth surface at a perpendicular angle, it reflects back in the same direction, creating a clear and bright image. However, when light strikes a rough or uneven surface, it reflects in many different directions, creating a blurred or diffused image.
The reflection of EMR can also affect the Earth’s energy balance. When radiation from the Sun strikes the Earth’s surface, some of it is reflected back into space, reducing the amount of energy that is absorbed by the Earth’s atmosphere and surface. This reflection can be influenced by the properties of the surface, such as its color, texture, and composition.
Reflection from surfaces occurs in two ways:
Specular Reflection
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. It gives rise to images.
Specular reflection is a type of reflection in which electromagnetic radiation (EMR) is reflected off a smooth surface at an angle that is equal to the angle of incidence, creating a clear and sharp image. This type of reflection occurs when the surface is very smooth and the EMR interacts with it as if it were a mirror.
Specular reflection is different from diffuse reflection, which occurs when EMR is reflected off a rough or uneven surface and is scattered in many different directions, creating a blurred or diffused image.
Specular reflection can be observed in many natural and artificial processes. For example, when light from the Sun strikes the surface of a still body of water at a particular angle, the reflection can create a clear and sharp image of the Sun on the water’s surface. Similarly, mirrors and other smooth surfaces can reflect light and other types of EMR in a specular manner.
Diffuse Reflection
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.
Diffuse reflection is a type of reflection in which electromagnetic radiation (EMR) is reflected off a rough or uneven surface and is scattered in many different directions, creating a blurred or diffused image. This type of reflection occurs when the surface is not smooth, and the EMR interacts with the surface irregularities, causing it to reflect in many different directions.
Diffuse reflection is different from specular reflection, which occurs when EMR is reflected off a smooth surface at an angle that is equal to the angle of incidence, creating a clear and sharp image.
Diffuse reflection can be observed in many natural and artificial processes. For example, when light from the Sun strikes the surface of a rocky mountain, the reflection can create a diffused and scattered image of the Sun. Similarly, when light strikes a textured or matte surface, like a wall or a piece of paper, the reflection appears diffused and less clear than a reflection off a smooth, glossy surface.
Spectral Interaction of Vegetation and water
If the wavelengths are much smaller than the surface variations or the partic
If the wavelengths are much smaller than the surface variations or the particle sizes that make up the surface, the 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.
Vegetation and water interact with electromagnetic radiation (EMR) in different ways across the electromagnetic spectrum. These interactions can be used to remotely sense and monitor vegetation and water properties, such as vegetation health, biomass, and water quality.
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).
Vegetation refers to the collection of plant life in a particular area, ranging from small mosses and grasses to large trees and shrubs. Vegetation is an essential component of ecosystems, providing important services such as food and habitat for animals, oxygen production, carbon sequestration, and erosion control.
Vegetation interacts with its environment in many ways, including through the uptake of nutrients and water from the soil, the absorption of sunlight for photosynthesis, and the release of oxygen and water vapor through transpiration. The growth and productivity of vegetation can be influenced by a wide range of factors, such as temperature, rainfall, soil nutrients, and disturbances like wildfires and land use changes.
Vegetation can be monitored and studied using a variety of techniques, including field measurements, remote sensing, and modeling. Remote sensing techniques, in particular, have proven to be a powerful tool for mapping and monitoring vegetation across large areas and at different spatial and temporal scales. For example, satellite imagery can be used to estimate vegetation cover, biomass, and health, as well as to detect changes in vegetation over time due to factors like climate change and land use changes.
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 wavelength.
Water is a vital resource for life on Earth, and it plays an important role in many natural and human systems. Water is essential for the survival of plants and animals, and it also supports many ecosystem services, such as nutrient cycling, erosion control, and flood regulation.
Water exists in different forms on Earth, including oceans, rivers, lakes, groundwater, and atmospheric moisture. The distribution and availability of water vary widely across different regions and ecosystems, and it is often a limiting factor for the growth and productivity of vegetation and the survival of animals.
Water is also an important resource for human societies, used for a wide range of purposes such as drinking, irrigation, energy production, and industrial processes. The management of water resources is therefore a key challenge for ensuring the sustainability of human societies and the environment.