What is Light: history, nature, behavior, propaganda

Light is an electromagnetic wave that can be captured by the sense of sight. It forms part of the electromagnetic spectrum: called visible light. Over the years, various theories have been proposed to explain its nature.

For example, it has long been believed that light consists of objects or streams of particles emitted from the eyes of observers. This belief of the Arabs and the ancient Greeks was shared by Isaac Newton (1642-1727) to explain the phenomena of light.

Although Newton suspected that light had wave properties, and Christian Huygens (1629-1695) succeeded in explaining refraction and reflection with wave theory, the belief in light as a particle was widespread among scientists until the early 19th century.

The sky is blue because of the scattering of sunlight in the atmosphere.

At the beginning of that century, the English physicist Thomas Young proved beyond doubt that rays interfere with each other, just like mechanical waves in strings.

This could only mean that light is a wave, not a particle, but no one knew what kind of wave it was until James Clerk Maxwell called it an electromagnetic wave in 1873.

In 1887, supported by the experimental results of Heinrich Hertz, the wave nature of light was established as a scientific fact.

But at the beginning of the 20th century, new evidence appeared about the corpuscular nature of light. This nature exists in emission and absorption phenomena, where light energy is carried in packets called “photons.”

Thus, since light propagates as a wave and interacts with matter like a particle, a dual nature is now recognized: wave-particle.

Article index

1 The nature of light
2 The behavior of light

  • Huygens principle
  • Fermat principle
  • Diffusion of light


  • Diffraction
  • Interference and polarization

4 Phenomena of light
4.1 Reflection
4.2 Fracture
4.3 Dispersion
5 Theories about light
5.1 Aristotelian theory
5.2 Newton’s corpuscular theory
5.3 Huygens wave theory
5.4 Maxwell’s electromagnetic theory
5.5 Einstein’s corpuscular theory
6 Literature

The nature of light
Understand that the nature of light is twofold, that it propagates as an electromagnetic wave, and that its energy comes in photons.

They are massless and move at a speed of 300,000 km/s in a vacuum. This is the speed of light in a vacuum, but light can travel through other media, but at different speeds.

When photons reach our eyes, light sensors are activated. Information is transmitted to the brain and interpreted there.

When a source emits a number of photons, we see it as a light source. If, on the other hand, it emits less, it is interpreted as an opaque source. Each photon has an energy that it interprets as its brain. water, blue photons are more energetic than red photons.

Any source of light emits photons of different energies, hence its color.

If something emits photons of one type of energy with another, it is called monochromatic light. A laser is a good example of monochromatic light. Finally, the distribution of photons in the source is called the spectrum.

Being a wave is also a long wave. As mentioned, light is part of the electromagnetic spectrum, which covers a very wide range of wavelengths, from radio waves to gamma rays. The image below shows how a beam of white light is scattered by a triangular prism. Light wavelengths are divided into long (red) and short (blue) wavelengths.

The behavior of light

Light has two behaviors, wave and particle. Light travels like an electromagnetic wave, so it can carry energy. But when light interacts with matter, it behaves like a beam of particles called photons.

In 1802, physicist Thomas Young (1773-1829) showed that light behaves. using a wave double burst experiment.

Thus, he was able to make maximum and minimum interventions on the screen. This behavior is characteristic of waves, so Young was able to show that light is a wave and measure its wavelength.

The other side of light is a particle, represented by packets of energy called photons, moving in a vacuum with speed c = 3 x 108 m/s and is massless. But they have energy AND:

E = hf

And also the moment of magnitude:

p = E / c

Where h is Planck’s constant, whose value is 6.63 x 10-34 Joules per second, and F is the frequency of the wave. Combine these words:

p = hf / c

And since wavelength λ and frequency related c = λ.f, it remains:

p = h / λ → λ = h / p

Huygens principle

There are two important principles to consider when studying the behavior of light: Huygens’ principle and Fermat’s principle. Huygens’ principle states:

Any point on the wavefront acts as a point source, which in turn produces secondary spherical waves.

Why spherical waves? If we assume that the medium is homogeneous, the light emitted by a point source will spread equally in all directions. We can imagine light spreading in the middle of a large sphere of uniformly distributed rays. Whoever observes this light realizes that it travels in a straight line towards the eye and travels perpendicular to the wave front.

If the light rays come from a very distant source, such as the Sun, the wave front is flat and the rays are parallel. This approximation is geometrical optics.

Farm principle
The farm principle says:

A light ray traveling between two points follows a path that takes the least amount of time.

This principle owes its name to the French mathematician Pierre de Fermat (1601-1665), who first established it in 1662.

According to this principle, light travels at a constant speed in a homogeneous medium, so it has a uniform rectilinear motion and its trajectory is a straight line.

Diffusion of light
Light propagates like an electromagnetic wave. Both the electric field and the magnetic field induce each other and form coherent waves that are in phase and perpendicular to each other and to the direction of propagation.

In general, a wave that propagates in space is a wave front. It is a set of points that have the same amplitude and phase. According to Huygens’ principle, by knowing the location of a wave front at one moment, one can know any location next.


The wave behavior of light is clearly demonstrated by two important phenomena that occur during its propagation: diffraction and interference. Internal DiffractionWhether it’s water, sound, or light, waves are distorted when they pass through holes, around obstacles, or around corners.

If the aperture is large relative to the wavelength, the distortion is less, and if the aperture is smaller, the waveform change is more noticeable. Diffraction is a special property of waves, so when light exhibits diffraction, we know that it has wave behavior.

Interference and polarization

In turn, the interference of light occurs when the electromagnetic waves that make them are aligned with each other. In doing so, they add vectorially and this can cause two types of interference:

–Constructive, when the intensity of the resulting wave is greater than the intensity of the components.

Destructive if the intensity is less than the components.

Light wave interference occurs when the waves are monochromatic and always maintain the same phase difference. This is called compatibility. Such light may come from, for example, a laser. Common sources such as incandescent lamps do not emit coherent light because the light emitted by the millions of atoms in the filament is constantly changing phase.

But if an opaque screen with two small holes close to each other is placed on the same bulb, the light from each cell acts as a coherent source.

Finally, when the electromagnetic field oscillates all in the same direction, Polarization. Natural light is not polarized because it consists of many components that vibrate in different directions.

Yang’s experiment

At the beginning of the 19th century, the English physicist Thomas Young was the first to obtain coherent light with a conventional light source.

In his famous double-hole experiment, he passed light through a hole in an opaque screen. According to the Huygens principle, secondary sources appear, which in turn are passed through a second opaque screen with two holes.

The light thus obtained illuminated the wall in the dark room. What is visible is a pattern of alternating light and dark areas. The existence of this pattern is explained by the phenomenon of interference described above.

Yang’s experiment was very important because it revealed the wavelike nature of light. Later experiments were conducted with elementary particles such as electrons, neutrons, and protons with similar results.

Phenomena of light

When a light beam hits a surface, some of the light may be reflected and some may be absorbed. If it is a transparent medium, some light will continue to pass through it.

Also, the surface may be smooth, mirror-like, or rough and uneven. Reflection that occurs on a smooth surface is called specular reflection, otherwise it is diffuse reflection or irregular reflection. A highly polished surface such as a mirror can reflect up to 95% of incident light.

Clear reflection

The image shows a beam of light traveling through a medium that may be air. The event with Angle1 is reflected in the flat specular surface and Angle2. The line labeled normal is perpendicular to the surface


What is Light: history, nature, behavior, propaganda

Refraction of light occurs because light travels at different speeds depending on the medium. In a vacuum, the speed of light is c = 3 x 108 m/s, but when light reaches a material medium, absorption and emission processes occur that reduce the energy and its speed.

For example, when moving in air, light is almost equal to s, and in water, light travels three-quarters of the speed. c, in glass it does so about two-thirds of the way c.

Refractive index

The refractive index is denoted p and is defined as the ratio between the speed of light in a vacuum v and its average speed v:

n = c / v

The speed of light in a vacuum is always greater than in a material medium, so the refractive index is always greater than 1. Some typical values of N are:

  • Air: 1.0003
  • Water: 1.33
  • Glass: 1.5
  • Diamond: 2.42

Snell’s law

When a light beam obliquely strikes the boundary between two media, such as air and glass, part of the light is reflected and part passes through the glass.

In this case, the wavelength and speed change from one medium to another, but not the frequency. since v = c / n = λ.f and also in space c = .o. F, then we have:

(λor.f / n) = λ.f → λ = λor/ n

In other words, the wavelength in a given medium is always less than the wavelength in vacuum. λo

What is Light: history, nature, behavior, propaganda

Look at the triangles with a common hypotenuse in red. At each location, the hypotenuse measures λ1/ senθ1 and λ2/ senθ2, respectively, and the proportional changes in λ:

λ1/ sin θ1 = λ2/ sin θ2

What λ = λor/ n you need:

(λor/ n1) / sen θ1 = (λor/ n2) / sen θ2

Which can be lowered:

p1. sen θ1 = n2 .sen θ2

It is a formula for Snell’s Law, named after the Dutch mathematician Willebord Snell (1580-1626), who derived it experimentally by observing light passing from air to water and glass.

Alternatively, Snell’s law defines the test index, written in terms of the speed of light in each region: n = c / v:

(c / v1). sen θ1 = (c / v2).sen θ2

v2. sen θ1 = v1 .sen θ2


What is Light: history, nature, behavior, propaganda

As mentioned above, light consists of photons of different energies, and each energy is perceived as a color. White light contains photons of all energies, so they can be split into different colors of light. This is the scattering of light that was already studied by Newton.

Newton took an optical prism through which he passed a beam of white light and obtained bands of colors from red to violet. This period is the spectrum of visible light in Figure 2.

Scattering of light is a natural phenomenon, and we admire the sky when a rainbow appears. Sunlight hits water droplets in the atmosphere that act as tiny Newtonian prisms, scattering the light.

The blue color you see in the sky is also a result of scattering. Rich in nitrogen and oxygen, the atmosphere emits mainly blue and violet colors, but the human eye is more sensitive to blue, so we see the sky in this color.

When the sun is low on the horizon, at sunrise or sunset, the sky turns orange because the light rays have to pass through a thick layer of atmosphere. The violet tones of lower frequencies interact less with the elements of the atmosphere and are used to reach the earth’s surface directly.

Atmospheres rich in dust and pollution, such as those in some large cities, have a gray color due to the dispersion of low frequencies.

Theories about light
Light is primarily thought of as a particle or wave. The corpuscular theory advocated by Newton viewed light as a beam of particles. And reflection and refraction, as Huygens said, can be adequately explained by considering light as a wave.

But long before these amazing scientists, people speculated about the nature of light. Among them, the Greek philosopher Aristotle cannot be absent. Here is a summary of the theories of light over time:

Aristotelian theory
2,500 years ago, Aristotle argued that light emanates from the observer’s eye, illuminates objects, and somehow returns as an image.

Newton’s corpuscular theory
Newton believed that light consists of small particles that travel in a straight line in all directions. When it reaches the eyes, it registers the sensation as light.

Huygens wave theory
Huygens’ treatise on light, in which he published his work, stated that it is a disturbance of the medium similar to sound waves.

Maxwell’s electromagnetic theory
Although the double-slit experiment left no doubt about the wave nature of light, until Maxwell in his electromagnetic theory stated that light consists of the propagation of an electromagnetic field, speculations about its wave form were made for much of the 19th century. .

Light as an electromagnetic wave explains the phenomena of light propagation described in previous chapters and is a concept accepted by modern physics, as is the corpuscular nature of light.

Einstein’s corpuscular theory
According to the current understanding of light, it consists of massless and uncharged particles called photons. Although they have no mass, as mentioned above, they have momentum and energy. This theory satisfactorily explains the interaction of light with matter, exchanging energy in discrete (quantized) quantities.

The photoelectric effect, proposed by Albert Einstein to explain the existence of light quanta, was discovered a few years earlier by Heinrich Hertz. The photoelectric effect consists of the emission of electrons by a substance exposed to some form of electromagnetic radiation, almost always in the ultraviolet to visible light range.

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