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.
There are three ways in which the total incident energy will interact with earth's surface materials. These are
Absorption (A) occurs when radiation (energy) is absorbed into the target while transmission (T) occurs when radiation passes through a target. Reflection (R) occurs when radiation "bounces" off the target and is redirected.
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.
• In remote sensing, we are most interested in measuring the radiation reflected from targets.
Reflection from surfaces occurs in two ways:
Most surface features of the earth lie somewhere between perfectly specular or perfectly diffuse reflectors. Whether a particular target reflects specularly or diffusely, or somewhere in between, depends on the surface roughness of the feature in comparison to the wavelength of the incoming radiation.
Let's take a look at a couple of examples of targets at the Earth's surface and how energy at the visible and infrared wavelengths interacts with them.
A chemical compound in leaves called chlorophyll strongly absorbs radiation in the red and blue wavelengths but reflects green wavelengths.
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.
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.
• 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.
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Department of Geology
Aligarh Muslim University, Aligarh - 202 002 (India)