In this article, we will discuss how electromagnetic radiation interacts with objects present on earth surface. EMR interaction with earth surface is one of the main component of remote sensing.
The electromagnetic radiation reaching the earth surface interacts with various objects such as vegetation, land surface, buildings etc., present on the earth surface. Depending on the physical properties of object, wavelength of EMR different types of interactions takes place.
Fundamental principle based on which remote sensing is performed depends on the interaction behavior. the principle can be stated as -“Different objects interact differently with different wavelengths of electromagnetic energy”.
Following are 3 important types of interaction that takes place between objects and electromagnetic radiation.
The energy incident on earth surface is equal to sum of energies used in reflection, absorption and transmission.
E(I) = E(R) + E(A) + E(T)
Where, E(I) = Energy Incident on Surface, E(A) = Energy absorbed by the surface, E(R) = Energy reflected by the surface and E(T) = Energy transmitted from surface into another object.
It is the process where energy is redirected into other directions after incident on surface of the object. The angle of incidence is equal to angle of reflection in case of smooth surfaces. However, if surface is not smooth energy is not redirected into a single direction, instead scatters into multiple directions.
Scattering at surface is special case of reflection and is dependent on surface properties of the object.
Reflection is very important phenomenon in remote sensing process. The reflected energy from the objects is recorded by the sensors. Since, each object reflects energy uniquely (or different objects reflect differently) they are distinguished with remote sensing process. However, the magnitude of energy reflected by an object at different wavelengths alters, hence 2 objects may be reflecting same amount of energy in a particular band. However, they can be distinguished using the images captured in other bands of data.
Example: Water and vegetation have similar amount of reflectance in visible band, however in infra red band water has zero reflectance. Hence, they can be distinguished very easily in that band.
As mentioned above based on roughness of surface reflection taking place at the object changes. It is classified into 2 types.
- Specular reflection
- Diffused reflection
Some portion of the incident energy is stored by the objects. This energy is not available for reflection. It is either released back into the atmosphere at a later time in the form of infrared waves or it gets transmitted to the other objects.
Depending on type of object, some times energy is transmitted into other objects. During transmission of energy, the wavelength, velocity and other properties of EMR may change.
What type of interaction takes place at an object is purely dependent on nature of the surface and physical properties of the object. Some times all incident energy on the object is reflected back into the atmosphere. While in some other cases, the entire incident energy on the object is absorbed by the object. If an object absorbs all incident energy it is called black body.
Spectral Reflectance Curve:
The response of an object towards incident electromagnetic radiation in the form of reflection is plotted as a graph to study the properties of object using remote sensing. This also helps in understanding at what wavelength remote sensing has to be performed to distinguish certain type of features.
The graph drawn between wavelength of incident electromagnetic radiation and percentage reflectance at the same wavelength is termed as spectral reflectance curve.
These curves are very useful in remote sensing studies and are unique for each and every object. Hence, these are also called as spectral signatures and are used to understand the behavior of objects and perform remote sensing.
More about the spectral reflectance curves of different features is covered in the other articles.