What is a Wave?

Image description: Electromagnetic waves are formed by the vibrations of electric and magnetic fields. These fields are always perpendicular to each other but also perpendicular to the direction of wave movement. Once formed, this energy travels at the speed of light until the next interaction with matter.

A Wave is Energy Propagating!

Saying that a wave is the propagation of a disturbance in the medium is not enough to understand what a wave is.
Saying that speed, wavelength, and frequency are the 3 properties characterizing a wave is not enough to understand what a wave is.
Saying that a wave moves with a determined speed that depends on the characteristics of the propagation medium does not show us the wave.
Saying that a wave transports energy without transporting matter still tells us nothing about the nature of a wave.

Now, imagine a rope that is shaken with a vertical gesture. This simple movement will draw a wave of a certain height, which seems to propagate forward, but in reality, no point of the rope moves; each point goes back and forth, moving vertically. This amplitude gives us an idea of the force with which we shake the rope but not of the wave itself. To understand what a wave is, we must remove the rope and imagine the air molecules undergoing the mechanical pressure of the rope. At that moment, only energy remains in the medium (the air), a force generated by the movement of the rope. This energy causes the air pressure to oscillate around an equilibrium value, the pressure increases and decreases alternately around this value.

The wave (i.e., the energy) displaces the air molecules without transporting them. As long as we shake the rope, we can measure the distance between 2 peaks or 2 troughs; this distance is the wavelength measured in meters. The rhythm with which we shake the rope corresponds to the frequency of the wave measured in Hertz. The speed of the wave measured in meters/second is equal to the wavelength multiplied by the frequency.

The Wave and Its Field

A wave propagates in a stable medium, capable of returning to a state of equilibrium; for a sound wave, it is the air pressure that moves relative to an average value. For an electromagnetic wave, it is the intensity of the electromagnetic field that moves relative to an average field value.

Electricity can be static, like amber, which, after being rubbed, attracts small objects. Magnetism can also be static, like in a magnet. But when these fields move together, they become self-propagating transverse electromagnetic waves.

Electromagnetic waves are formed when an electric field couples with a magnetic field. At that moment, they oscillate at right angles to each other and propagate perpendicularly to the direction of movement. In other words, the vibrations of the magnetic field and the electric field are always perpendicular to each other but also perpendicular to the direction of the wave.

The two types of waves, mechanical and electromagnetic, are two ways of transporting energy in a medium. Waves in water and sound waves in air are two examples of mechanical waves. This energy transport disturbs or vibrates matter (solid, liquid, gas, or plasma) without transporting it; water or air molecules collide but remain in the same place.

A variable magnetic field induces a variable electric field and vice versa; the two are intimately linked. These two fields, described by James Clerk Maxwell (1831-1879), when coupled, form electromagnetic waves (see image). Unlike mechanical waves, electromagnetic waves do not need a physical medium to propagate; they travel everywhere, even in the vacuum of space. Light, electromagnetic waves, and all radiations are derived from the same physical phenomenon: Electromagnetic Energy.

The field is a fundamental concept in physics; it is not made of anything else; it is itself that constitutes the real world. When a force acts on them, fields transport energy, from atoms to large galactic structures.

N.B.: The wave is said to be transverse if the energy moves perpendicularly to the direction of wave movement (movement of the arm shaking the rope or the energy of a pebble falling into the water). The wave is said to be longitudinal if the energy moves in the direction of wave movement (the speaker magnet). A wave can be both longitudinal and transverse (a stick striking a drum).

Frequency, Wavelength, and Energy

Frequency, wavelength, and energy are mathematically related; knowing one of these three values is enough to calculate the other two.

Radio waves and microwaves are generally described in terms of frequency (in hertz), infrared and visible light in terms of wavelength (in meters), and X-rays and gamma rays in terms of energy (electronvolts).

The frequency of the wave is the number of periodic phenomena or the number of peaks that reproduce per second, according to Heinrich Hertz (1857-1894), who established the existence of radio waves. From radio waves to gamma rays, the frequency is measured from a few Hertz to 10²⁶ Hertz.

The wavelength is the distance between two peaks. The longest waves (radio waves) can measure several kilometers, while the shortest waves (gamma waves) can measure up to 10^-12 meters (size of the atomic nucleus).

The energy of an electromagnetic wave is measured in electronvolts (eV). An electronvolt is the amount of kinetic energy needed to move an electron through a voltage potential of 1 volt. The lowest energies are those of radio waves (a few eV), while the highest energies are those of gamma rays (beyond 100 keV).

N.B.: Between wavelength (λ) and frequency (ν) there is the following relationship: ν = c / λ
ν = wave frequency in hertz, c = speed of light in vacuum in m/s, λ = wavelength in meters.

What is Light?

According to the equations of James Clerk Maxwell (1831-1879), light is a self-propagating transverse electromagnetic wave with electric and magnetic components where the electric and magnetic fields oscillate at right angles to each other and propagate perpendicularly to the direction in which they move indefinitely unless absorbed by intermediate matter.
In other words, each type of field - electric and magnetic - generates the other to propagate the entire composite structure in empty space at the finite speed of light.