LIGHT: HISTORY, NATURE, BEHAVIOR, PROPAGATION - PHYSICAL - 2025

The light is an electromagnetic wave can be detected by the sense of sight. It constitutes a part of the electromagnetic spectrum: what is known as visible light. Over the years, various theories have been proposed to explain its nature.

For example, the belief that light consisted of a stream of particles emitted by objects or by the eyes of observers was long held. This belief of the Arabs and the ancient Greeks was shared by Isaac Newton (1642-1727) to explain the phenomena of light.

Figure 1. The sky is blue thanks to the scattering of sunlight in the atmosphere. Source: Pixabay.

Although Newton came to suspect that light had wave qualities and Christian Huygens (1629-1695) managed to explain refraction and reflection with a wave theory, the belief of light as a particle was widespread among all scientists until the beginning of the 19th century..

At the dawn of that century, the English physicist Thomas Young showed without a doubt that light rays can interfere with each other, just as mechanical waves do in strings.

That could only mean that the light was a wave and not a particle, although no one knew what kind of wave it was until in 1873, James Clerk Maxwell claimed that light was an electromagnetic wave.

With the support of the experimental results of Heinrich Hertz in 1887, the wave nature of light was established as a scientific fact.

But at the beginning of the 20th century, new evidence emerged about the corpuscular nature of light. This nature is present in emission and absorption phenomena, in which light energy is transported in packages called “photons”.

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

Nature of light

It is clear that the nature of light is dual, propagating as an electromagnetic wave, whose energy comes in photons.

These, which have no mass, move in a vacuum with a constant speed of 300,000 km / s. It is the known speed of light in a vacuum, but light can travel through other media, albeit at different speeds.

When the photons reach our eyes, the sensors that detect the presence of light are activated. The information is transmitted to the brain, and interpreted there.

When a source emits a large number of photons, we see it as a bright source. If, on the contrary, it emits few, it is interpreted as an opaque source. Each photon has a certain energy, which the brain interprets as a color. For example blue photons are more energetic than red photons.

Any source generally emits photons of different energies, hence the color with which it is seen.

If nothing else emits photons with a single type of energy, it is called monochromatic light. The laser is a good example of monochromatic light. Finally, the distribution of photons in a source is called a spectrum.

A wave is also characterized by having a certain wavelength. As we have said, light belongs to the electromagnetic spectrum, which covers an extremely wide range of wavelengths, from radio waves to gamma rays. The following image shows how a beam of white light scatters a triangular prism. Light is separated into long (red) and short (blue) wavelengths.

In the middle is the narrow band of wavelengths known as the visible spectrum, ranging from 400 nanometers (nm) to 700 nm.

Figure 2. The electromagnetic spectrum showing the range of visible light. Source: Source: Wikimedia Commons. Author: Horst Frank.

Behavior of light

Light has dual, wave and particle behavior as examined. Light propagates in the same way as an electromagnetic wave, and as such, it is capable of transporting energy. But when light interacts with matter, it behaves like a beam of particles called photons.

Figure 4. Propagation of an electromagnetic wave. Source: Wikimedia Commons. SuperManu.

In 1802, the physicist Thomas Young (1773-1829) demonstrated that light had a wave behavior using the double slit experiment.

In this way he was able to produce maximum and minimum interference on a screen. This behavior is typical of waves and thus Young was able to demonstrate that light was a wave and also was able to measure its wavelength.

The other aspect of light is that of a particle, represented by packets of energy called photons, which in a vacuum move with speed c = 3 x 10 ⁸ m / s and have no mass. But they do have E energy:

And also momentum of magnitude:

Where h is Planck's constant, whose value is 6.63 x 10 ^-34 Joule.second and f is the frequency of the wave. Combining these expressions:

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

Huygens principle

Figure 5. Wave front and light rays propagating in a straight line. Source: Serway. R. Physics for Science and Engineering.

When studying the behavior of light, there are two important principles to consider: Huygens' principle and Fermat's principle. Huygens' principle states that:

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

If the light rays come from a very distant source, for example the Sun, the wave front is flat and the rays are parallel. This is what the geometric optics approach is all about.

Fermat's principle

Fermat's principle states that:

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

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

Propagation of light

Light travels like an electromagnetic wave. Both the electric field and the magnetic field generate each other, constituting coupled waves that are in phase and are perpendicular to each other and to the direction of propagation.

In general, a wave propagating in space can be described in terms of the wave front. This is the set of points that have equal amplitude and phase. Knowing the location of the wavefront at a given instant, any subsequent location can be known, according to Huygens' principle.

Diffraction

Laser diffracted by a hexagonal slit. Lienzocian

The wave behavior of light is clearly manifested in two important phenomena that arise during its propagation: diffraction and interference. In diffraction, waves, be they those of water, sound or light, are distorted when they pass through openings, go around obstacles, or go around corners.

If the aperture is large compared to the wavelength, the distortion is not very large, but if the aperture is small, the change in waveform is more noticeable. Diffraction is an exclusive property of waves, so when light exhibits diffraction we know that it has wave behavior.

Interference and polarization

For its part, the interference of light occurs when the electromagnetic waves that compose them overlap. When doing so, they are added vectorially and this could give rise to 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 that of the components.

Light wave interference occurs when the waves are monochromatic and maintain the same phase difference all the time. This is called consistency. A light like this can come from a laser for example. Common sources such as incandescent bulbs do not produce coherent light because the light emitted by the millions of atoms in the filament is constantly changing phase.

But if an opaque shade with two small openings close to each other is placed on that same light bulb, the light that comes out of each slot acts as a coherent source.

Finally, when the oscillations of the electromagnetic field are all in the same direction, polarization occurs. Natural light is not polarized, as it is made up of many components, each one oscillating in a different direction.

Young's experiment

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

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

Figure 6. Animation of Young's double slit experiment. Source: Wikimedia Commons.

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

Young's experiment was very important because it revealed the wave nature of light. Subsequently the experiment has been carried out with fundamental particles such as electrons, neutrons and protons, with similar results.

Phenomena of light

Reflection

Reflection of light in the water

When a ray of light hits a surface, some of the light can be reflected and some absorbed. If it is a transparent medium, some of the light continues its way through it.

Also, the surface can be smooth, like a mirror, or rough and uneven. The 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 the incident light.

Specular reflection

The figure shows a ray of light traveling in a medium, which may be air. It falls at angle θ ₁ on a plane specular surface and is reflected at angle θ ₂. The line denoted normal is perpendicular to the surface.

The angle of incidence is equal to the angle of reflection. Source: Serway. R. Physics for Science and Engineering.

Both the incident and the reflected ray and the normal to the specular surface are in the same plane. The ancient Greeks had already observed that the angle of incidence equals the angle of reflection:

This mathematical expression is the law of reflection of light. However, other waves such as sound for example, are also capable of reflection.

Most surfaces are rough, and therefore light reflection is diffuse. In this way the light they reflect is sent to all directions, so objects can be seen from anywhere.

Since some wavelengths are reflected more than others, objects have different colors.

For example, the leaves of trees reflect light that is approximately in the middle of the visible spectrum, which corresponds to the color green. The rest of the visible wavelengths are absorbed: ultraviolet near blue (350-450 nm) and red light (650-700 nm).

Refraction

Refraction phenomenon. Josell7

The 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 10 ⁸ m / s, but when light reaches a material medium, absorption and emission processes arise that cause the energy to decrease, and with it the speed.

For example, when moving in air, light travels with almost equal speed to c, but in water, light travels at three-quarters of c, while in glass it travels at approximately two-thirds of c.

Refractive index

The refractive index is denoted n and defined as the quotient between the speed of light in a vacuum c and its speed in said medium v:

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

-Air: 1.0003

-Water: 1.33

-Glass: 1.5

-Diamond: 2.42

Snell's Law

When a ray of light strikes the border between two media obliquely, such as air and glass for example, part of the light is reflected and another part continues its way into the glass.

In this case, the wavelength and the speed undergo a variation when passing from one medium to another, but not the frequency. Since v = c / n = λ.f and also in vacuum c = λo. f, then we have:

That is, the wavelength in a given medium is always less than the wavelength in vacuum λo.

Figure 8. Snell's Law. Source: Left figure: diagram of the refraction of light. Rex, A. Fundamentals of Physics. Right figure: Wikimedia Commons. Josell7.

Note the triangles that have a common hypotenuse in red. In each medium, the hypotenuse measures λ ₁ / sin θ ₁ and λ ₂ / sin θ ₂ respectively, since λ and v are proportional, therefore:

Since λ = λ _o / n we have:

Which can be expressed as:

This is the formula of Snell's law, in honor of the Dutch mathematician Willebrord Snell (1580-1626), who derived it experimentally by observing light passing from air to water and glass.

Alternatively, Snell's law is written in terms of the speed of light in each medium, making use of the definition of refractive index: n = c / v:

Dispersion

As explained above, light is made up of photons with different energies, and each energy is perceived as a color. White light contains photons of all energies and can therefore be broken down into different colored lights. This is the scattering of light, which had already been studied by Newton.

Drops of water in the atmosphere behave like small prisms. Source: Pixabay.

Newton took an optical prism, passed a beam of white light through it, and obtained colored stripes ranging from red to violet. This fringe is the spectrum of visible light seen in Figure 2.

The scattering of light is a natural phenomenon, the beauty of which we admire in the sky when the rainbow forms. Sunlight falls on water droplets in the atmosphere, which act like tiny Newton-like prisms, thus scattering the light.

The blue color with which we see the sky is also a consequence of dispersion. Rich in nitrogen and oxygen, the atmosphere disperses mainly shades of blue and violet, but the human eye is more sensitive to blue and therefore we see the sky of this color.

When the Sun is lower on the horizon, during sunrise or sunset, the sky turns orange thanks to the fact that the light rays must pass through a thicker layer of the atmosphere. The reddish tones of lower frequencies interact less with the elements of the atmosphere and take advantage to reach the surface directly.

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

Theories about light

Light has been considered primarily as a particle or as a wave. The corpuscular theory that Newton defended, considered light as a beam of particles. Whereas reflection and refraction could be adequately explained by assuming that light was a wave, as Huygens argued.

But long before these remarkable scientists, people had already speculated about the nature of light. Among them the Greek philosopher Aristotle could not be absent. Here is a brief summary of the theories of light over time:

Aristotelian theory

2,500 years ago Aristotle claimed that light emerged from the observer's eyes, illuminated objects, and returned in some way with the image so that it could be appreciated by the person.

Newton's corpuscular theory

Newton held the belief that light consisted of tiny particles that propagate in a straight line in all directions. When they reach the eyes, they register the sensation as light.

Huygens wave theory

Huygens published a work called Treatise on light in which he proposed that this was 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, for much of the nineteenth century there was speculation about the type of wave it was, until Maxwell stated in his electromagnetic theory that light consisted of the propagation of an electromagnetic field.

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

Einstein's corpuscular theory

According to the modern conception of light, it consists of massless and uncharged particles called photons. Despite not having mass, they have momentum and energy, as explained above. This theory successfully explains the way that light interacts with matter, by exchanging energy in discrete (quantized) quantities.

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

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