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The fuel of remote sensing

The application of remote sensing sensors and methods relies on the existence of electromagnetic (EM) radiation. Whether it is beeing measured through the emissions of an object (reflection of sunlight) or whether we actively send radiation from an instrument, without this source of energy, we cannot analyze a signal. But: What exactly are “waves”? You may think of a wave as a moving undulation transporting energy.

An example of natural wave development are water waves developing after you threw a stone into the water. Another kind of waves is sound waves developing while speaking or a police car passes you.
The radiation coming from the sun is travelling through so-called electromagnetic waves. Here, electrical and magnetic fields are coupled.


History excourse: EM waves

In the 1860’s and 1870’s, the Scottish scientist James Clerk Maxwell developed a scientific theory to explain EM waves. He noticed that electrical fields and magnetic fields can couple together to form electromagnetic waves. He summarized this relationship between electricity and magnetism into what are now referred to as “Maxwell’s Equations“.

Heinrich Hertz, a German physicist, applied Maxwell’s theories to the production and reception of radio waves. The unit of frequency of a radio wave — one cycle per second — is named the hertz, in honor of Heinrich Hertz.

His experiment with radio waves solved two problems. First, he had demonstrated in the concrete, what Maxwell had only theorized — that the velocity of radio waves was equal to the velocity of light. This proved that radio waves were a form of light. Second, Hertz found out how to make the electric and magnetic fields detach themselves from wires and go free as Maxwell’s waves — electromagnetic waves.


Structural view on an EM wave

As visualized on the right, an EM wave consists of an electrical field (EF, red) and a magnetic field (MF, blue). The EF varies in magnitude in the direction that is perpendicular to the that of the travel direction. A corresponding MF is oriented at right angles to the EF. Both fields travel at the speed of light (~300,000 km/s).

A travelling EM wave with an electric field (red) and a magnetic field (blue) (AWF-Wiki YEAR)

How do we describe a EM wave?

The terms light, electromagnetic waves, and radiation all refer to the same physical phenomenon: electromagnetic energy. This energy can be described by frequency, wavelength, or energy. All three are related mathematically such that if you know one, you can calculate the other two using this equation:

GLEICHUNG!

Thus, EM waves can be described through the characteristics of two parameters: a) wavelength and b) frequency. Both are closley related (inversely) and depend on each other.

Wavelength

Electromagnetic waves have crests and troughs similar to those of ocean waves. The distance between crests is refererred to as ‘wavelength’ and measured written as λ. The shortest wavelengths are just fractions of the size of an atom, while the longest wavelengths scientists currently study can be larger than the diameter of our planet.

Frequency

The number of crests that pass a given point within one second is described as the frequency of the wave. One wave or cycle—per second is called a Hertz (Hz), after Heinrich Hertz who established the existence of radio waves. A wave with two cycles that pass a point in one second has a frequency of 2 Hz.

Explore the unique features of an EM wave

This interactive tool gives you a hands-on experience of the frequency and the amplitude of a wave, two very important parameters. Try the sliders below to get a feeling for what these parameters actually mean.





The spectrum of EM radiation

Electromagnetic energy travels in waves and spans a broad spectrum from very long radio waves to very short gamma rays. The human eye can only detect only a small portion of this spectrum called visible light. A radio detects a different portion of the spectrum, and an x-ray machine uses yet another portion. Scientific instruments use the full range of the electromagnetic spectrum to study the Earth, the solar system, and the universe beyond.

A scheme of the electromagnetic spectrum with indication of wavelengths, frequencies and energies (ESA 2019).

The graphic above demonstrates the different portions of the electromagnetic spectrum, including their names, wavelengths and frequencies. It also depicts the fields of application and how our life is influenced by them. Furthermore, it shows the atmospheric windows at which the electromagnetic radiation grants us a peek through the atmosphere of the Earth.