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Table of contents
When electromagnetic radiation (EMR) reaches Earth’s surface, it interacts with the target in various ways. These interactions are very much important in remote sensing, helping us gather valuable information about the terrain and materials. The interaction of EMR with the target occurs primarily in three forms: absorption, transmission, and reflection. Each interaction depends on the wavelength of the energy and the material’s properties.
The Basics of Interaction of EMR with Target
The amount of radiation that strikes, reflects, or passes through a surface is measured in radiant flux (Φ), usually expressed in watts (W). Understanding the behavior of radiant flux as it interacts with different surfaces is essential in remote sensing. By observing how incoming EMR behaves when it hits a surface, we can gather significant information about the target.
The interaction of EMR with the target follows the basic radiation budget equation:
Φiλ = ρλ + τλ + αλ
Where:
Φiλ is the incident radiant flux,
ρλ is the energy reflected from the surface,
αλ is the energy absorbed, and
τλ is the energy transmitted through the surface.
Forms of Interaction of EMR with Target
Absorption of EMR by the Target
When EMR is absorbed by a surface, it is transformed into heat, causing a rise in temperature. Hemispherical absorptance is the ratio of absorbed energy to the incident radiant flux, representing how much of the EMR is taken in by the target.
Transmission of EMR Through the Target
Some energy may pass through a surface, which is known as transmission. Hemispherical transmittance represents the ratio of transmitted energy to the incident radiant flux. This interaction is particularly important in analyzing transparent or semi-transparent materials.
Reflection of EMR from the Target
Reflection occurs when the EMR bounces off a surface. Hemispherical reflectance measures how much energy is reflected in comparison to the incident energy. Reflectance plays a key role in remote sensing, as different surfaces reflect EMR differently, allowing us to distinguish between various materials.
Specular vs. Diffuse Reflection in Interaction of EMR with Target
The nature of the surface determines how EMR is reflected. There are two main types of reflecting surfaces:
Specular Reflection: Occurs when the surface is smooth, and the EMR is reflected in a single direction. For example, a mirror or calm water body reflects EMR in this way.
Diffuse Reflection: Occurs when the surface is rough, causing the EMR to scatter in multiple directions. Diffuse reflection doesn’t form a mirror image but spreads light evenly, like on a rough terrain or sandy surface.
In some cases, a surface may behave as an ideal diffuse reflector or a Lambertian surface, where the reflected energy is uniformly distributed in all directions.
Importance of Hemispherical Measurements in Remote Sensing
Hemispherical measurements consider all energy within a hemisphere above the surface. This is crucial in studying diffuse reflection, where energy is scattered in multiple directions. In remote sensing, we are generally interested in hemispherical reflectance properties to gather accurate information about the terrain.
Specular reflection, on the other hand, is directional, meaning it reflects EMR in a specific direction. This type of reflection can complicate remote sensing because the sensor must be positioned precisely in the path of the reflected energy.
Conclusion
The interaction of EMR with the target is fundamental in remote sensing technology. By analyzing how EMR is absorbed, transmitted, or reflected from different surfaces, scientists can extract important data about the Earth’s surface. This knowledge is vital for applications such as environmental monitoring, resource management, and geographical mapping.
Understanding the behavior of EMR as it interacts with the target is key to making accurate interpretations in remote sensing.
Test Your Knowledge with MCQs
- Which of the following statements are TRUE about the interaction of EMR with a target?
- Absorption of EMR results in an increase in the target’s temperature.
- Transmission refers to the EMR passing through the target.
- Specular reflection occurs on rough surfaces and scatters EMR in multiple directions.
- Hemispherical reflectance measures the ratio of reflected energy to incident energy.
- Which of the following are forms of interaction of EMR with a target?
- Absorption
- Refraction
- Transmission
- Reflection
- Match the following terms with their definitions:
- Radiant flux (Φ) a. Ratio of absorbed energy to incident radiant flux
- Hemispherical absorptance b. Energy reflected from the surface
- ρλ c. Amount of radiation striking, reflecting, or passing through a surface
- τλ d. Energy transmitted through the surface
- Assertion: Hemispherical measurements are crucial in remote sensing.
Reason: Hemispherical measurements consider all energy within a hemisphere above the surface, which is important for studying diffuse reflection.
a) Both assertion and reason are true, and the reason is the correct explanation of the assertion.
b) Both assertion and reason are true, but the reason is not the correct explanation of the assertion.
c) Assertion is true, but the reason is false.
d) Assertion is false, but the reason is true.
- Assertion: Specular reflection can complicate remote sensing.
Reason: Specular reflection is directional, requiring precise sensor positioning.
a) Both assertion and reason are true, and the reason is the correct explanation of the assertion.
b) Both assertion and reason are true, but the reason is not the correct explanation of the assertion.
c) Assertion is true, but the reason is false.
d) Assertion is false, but the reason is true.
- Which of the following is NOT a form of interaction of EMR with a target?
a) Absorption
b) Diffraction
c) Transmission
d) Reflection - What is the term for the ratio of absorbed energy to the incident radiant flux?
a) Hemispherical reflectance
b) Hemispherical transmittance
c) Hemispherical absorptance
d) Radiant flux - Which type of reflection occurs on a smooth surface and results in EMR being reflected in a single direction?
a) Specular reflection
b) Diffuse reflection
c) Lambertian reflection
d) Isotropic reflection - What does the symbol Φiλ represent in the radiation budget equation?
a) Reflected energy
b) Absorbed energy
c) Transmitted energy
d) Incident radiant flux - Which type of surface reflects energy uniformly in all directions?
a) Specular reflector
b) Diffuse reflector
c) Lambertian surface
d) Both b and c
Answers:
- True, True, False, True
- True, False, True, True
- c, a, b, d
- a. Both Assertion and Reason are true, and Reason is the correct explanation for Assertion.
- a. Both Assertion and Reason are true, and Reason is the correct explanation for Assertion.
- b
- c
- a
- d
- d
FAQs
The primary forms of interaction are absorption, transmission, and reflection. Absorption occurs when EMR is absorbed by the target and converted into heat. Transmission happens when EMR passes through the target. Reflection occurs when EMR bounces off the target’s surface.
Specular reflection occurs on smooth surfaces, where EMR is reflected in a single direction (like a mirror). Diffuse reflection occurs on rough surfaces, causing EMR to scatter in multiple directions.
Lambertian surfaces are ideal diffuse reflectors that reflect energy uniformly in all directions. They serve as a reference for comparing the reflectance properties of real-world surfaces and are useful in calibrating remote sensing instruments and algorithms.
