Nature of Electromagnetic Radiation

The explanation of electromagnetic radiation (REM) begins with an understanding of the duality of the behavior of its nature: wave and energy. This means that REM that propagates through empty space, like sunlight, is at the same time a waveform and a form of energy. This concept of duality is extremely important for remote sensing because whenever someone is analyzing any type of image of remote sensing, the coexistence of electromagnetic radiation in the waveform and in the form of energy should be considered so that everything that is observed in the images with reference to the characteristics of the objects. The duality of the behavior of electromagnetic radiation, wave, and energy, is formulated by models called
undulatory (wave) and corpuscular (energy).

Source: United States Environmental Protection Agency | US EPA

Undulatory model

According to the wave model, REM can be explained as a sine wave and harmonic waveform. According to Maxwell’s formulations, an electrically charged particle generates an electric field around itself and the movement of that particle generates, in turn, a magnetic field. Both fields, electric and magnetic, act by vibrating orthogonally to each other and have the same amplitudes, that is, they reach their maximum at the same time. Field variations are caused by particle vibrations.

Source: Three different representations of the magnetic field of a cylindrical bar magnet. Left: Iron filings. Center: field lines. Right: compass needles. The shape and direction of the field was accurately computed in all three cases. wikimedia Commons, 2021.

When this particle is accelerated, the perturbations between the two fields repetitively propagate in a vacuum in a direction orthogonal to the direction of the electric and magnetic fields, as shown in Figure 1.1. These disturbances of the electric (E) and magnetic (M) fields are called electromagnetic waves. The length of electromagnetic radiation depends on how long the particle is accelerated, and the frequency ν of the radiation depends on the frequency of vibration of the particle. Thus, an electromagnetic wave is defined as the oscillation of the E and M fields, according to a harmonic wave pattern, that is, waves spaced repetitively in time.

Source: The Electromagnetic Radiation Spectrum Infographic with labels and commonly used or known frequencies and wavelengths. Duarte Farrajota Ramos, 2021.

These dynamic fields always occur together as inseparable patterns, so that neither pure electric field nor pure magnetic field of radiated waves will occur separately from each other. Electromagnetic waves propagate in a vacuum at the speed of light (c= 299,292.46 km/s or approximately 300,000 km/s). If the distance between two successive wave peaks is measured , the length or size of the wave is determined, which is symbolized by the Greek letter λ and expressed in the system of metric units.

Waves can have lengths on the order of billionths of a meter (cosmic rays) to dimensions of kilometers (radio waves). If we measure the number of wave peaks that pass through a fixed reference point in space, we can determine their frequency ν. As the wave propagation speed is constant, waves with shorter lengths have higher frequencies, that is, a greater number of waves passes through the reference point in a given time. Conversely, those with longer lengths have lower frequencies.

From classical physics, the following relationship between wavelength λ and frequency  expresses the law of wave behavior: By custom of usage, in remote sensing we always refer to electromagnetic radiation by its length and not by its frequency. For example, electromagnetic radiation equivalent to blue sunlight has a wavelength of 0.455 micrometers or μm (1micro or μ equals 10-6m) and red sunlight is a wavelength of 0.617 μm. As by definition remote sensing is a measure of the interaction of electromagnetic radiation with the surface of objects, according to the wave model, the characteristics of the images are explained taking into account the relationship between the wave size and the size of the object.

This mode of interaction is called macroscopic interaction, which will be discussed in more detail below. A good example to explain this behavior are the interactions of electromagnetic radiation that occur with clouds. The electromagnetic radiations of the visible and infrared waves of sunlight, which are 0.4 to 2.5 μm in length, have, on average, sizes smaller than the average size of the water vapor molecules that make up a cloud, which is of the order of 30 µm. Because of this huge difference between the size of the visible and infrared waves and the size of the clouds’ water vapor molecules, the REM incident on the clouds is blocked by these molecules and reflected back to space.

They can’t get through the cloud. Then, the sensor will register the intensity of the cloud’s reflectance, forming an image of the cloud and not the objects that are on the Earth’s surface. One of the best-known sensors that operate at the visible and early near-infrared wavelengths is the camera. It is well known that if on the day of the aerial survey there is any presence of clouds, the cloud will be present in the photo.

And what happens if the wavelength is greater than the diameter of gaseous particles in clouds, as is the case with electromagnetic waves of microwave lengths, with sizes on the order of 1 to 100 cm? To understand this we must consider that a cloud is nothing more than a heap of vapor particles, that is, a gas. In this case, only a tiny fraction of the dimension of the centimeter-sized wave is barred by the corresponding particle size, the rest of the wave passing through it. Therefore, the cloud is not capable of physically blocking the passage of the wave. Thus, the wave crosses the cloud and hits the objects on the earth’s surface, which they reflect back to the sensor, which records the images of these objects.

In this case, clouds can be said to be transparent to the relatively long wavelengths of microwaves. Sensors that work with microwaves are called radar. In radar images, even if the sky is completely covered by clouds, the image formed is a clean image, with the entire surface of the terrain appearing in it. Hence, the great use of radar sensors in areas with frequent cloud cover, such as the Brazilian Amazon region.

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