WHAT IS THE LIGHT? : HISTORY OF THE THEORIES ON THE NATURE OF LIGHT
(First published in "Revista Universitaria-UABC No. 50, April-June 2005) Lydia Alvarez Camacho Guadalupe
Institute of Engineering, UABC
Jesus M. Beltrones Siqueiros
Center Condensed Matter Science, UNAM
Illustrations: Ernesto Anguiano Israel Santillan
The Greeks asked how did we see?
How do we see? This is an interesting question for all human beings. Today
days, to answer, we are fortunate to have information
accumulated over thousands of years. Back in 500 BC, at the time of the heyday of Greek culture, this was not true. In that
its
form. This theory is called extramisión. Leucippus, however believed that the approach
occurred in the opposite direction. Emitting objects "something" that contained his form and color, and
falling on the eyes, which did nothing but catch it. This theory is called interference.
Since we can not see without light, it was assumed that these fumes could not exist in the dark but not
Empedocles, Leucippus could not say anything about its nature. However,
was something quite clear: the emanations or "rays" travel in a straight line. This makes
its spread can be studied using the laws of geometry. No wonder that
Euclid, the father of traditional or Euclidean geometry,write a book on the subject,
which establishes the basis of perspective, a technique still used today
designers and artists worldwide.
was many years later when he settled the old debate
extramisión against intrusion. The manager of this was Alhazen, Arab physician born in what is now Iraq. Taking
among other things that look directly at the sun hurts his eyes,
rightly concluded that the eyes are receivers and not transmitters. Also correct in explaining that an object receives
ambient light and scattered in all directions. In the absence of obstacles, this
scattered light propagates in the eye and the sensation of the object. If no light, objects can
A s the father of physics. However, their
theories of light, never had the same level as the rest of his work. Despite this, many think tanks, uncritical reading of the texts of Aristotle were replaced by the uncritical reading of the texts of Newton. This fact plays a significant role in the story we tell below.
Newton says the light is made up of particles
got Newton's laws explain many phenomena, it was easy to exaggerate and
think they could explain them all. In this context, Isaac Newton explained the nature of light, whereas
consists of small balls, in what is known as
corpuscular theory or issue. The movement and
ARRIERS should only stop some of these and the projection of light on a screen would be simply a geometric shadow, as indeed for big hurdles. However, there was no way to explain why small obstacles as light deviates markedly from the straight path, or why the light projected onto a screen a complex diffraction pattern. Experiment
Young. Why in the region illuminated by two beams of light, there are areas
completely dark?
In particular, what was especially uncomfortable was that series of bright spots and dark
appeared in very specific orders. An experiment conducted in 1803 by Thomas Young
qWe call on all wave directions. If
consider the x-axis is the level of water in equilibrium, Figure 4 could be a fairly realistic scheme
one of these waves. A point where the water reaches its maximum height is a ridge
and one where it reaches its minimum height is a valley. The maximum distance
the height of the water can move away from its equilibrium position is called amplitude. The distance between peak and valley and ridge or between valley is called wavelength. It is important to note that although the wave is spreading horizontally, the water never moves in this direction. Water moves up or down when it leaves its equilibrium position, but once the shock returns to its pOn the other hand, if the direction of motion of the particle and the direction of propagation are the same, as in (b), we have a longitudinal wave. Figure 4 can be performed without any change to any transverse wave, and a little imagination can also be used to represent a longitudinal wave. The sum of waves
presents some interesting phenomena. When two waves are
do not necessarily coincide with the crests of the ridges of the other. If they do, they say they are in phase. If two waves with equal amplitude are in phase, the result is a wave with an amplitude twice as large as the original. This is called constructive interference. If there is this coincidence is said that the waves are fuand nothing to disturb, as in the empty space, sound can not propagate. Huygens
says the light is made of waves
At the time of the Greeks, the two main theories were based on an analogy between sight and touch. Many centuries later, the analogy between vision and hearing prompted many reflections. The sight and sound have in common that they function over long distances. The wave nature of sound was already well established. It was inevitable question would not be too light a wave? A Dutchman named Christian Huygens Firmat answered this question.
The ripples in the water are the most typical example of a wave. When considering light as a wave, Christian Huygens could explain the stripes on the expYoung erimento as a result of constructive and destructive interference between light traveling through different paths. In the wave theory of Huygens, the light intensity is related to the amplitude of the wave while the different colors correspond to different values of wavelengths, the color red has the longest wavelength, while the color Violet has the shortest. According to Huygens' principle, when light encounters an obstacle, every point it becomes a new source of wavelets that spread in all directions, hence no diffraction problems. These wavelets interfere with each other, resulting in preferential directions of propagation, such as those occurring in the phenomena of reflectionng and refraction.
The bands that appear in Young's experiment can be explained
directly as a result of constructive and destructive interference between two beams of light.
the same way, light and dark regions in a diffraction pattern,
explained as a result of interference between rays diffracted in different directions. More importantly, the Huygens principle to calculate, with extreme precision, the expected diffraction patterns for different obstacles.
In summary, we have that the wave theory can explain properties of light as
the intensity and color and phenomena such as reflection, refraction, as does the particle theory
. But alsocan not propagate in vacuum as the light it can?
Huygens's principle can explain this without problems, since it predicts that the diffraction
only be noticeable when the obstacle dimensions are similar to the wavelength. The wavelength of light is very small compared to
everyday objects, so in our daily life is very difficult to perceive it and therefore we
consider that light travels in straight lines. In contrast, the wavelength of the sound is much bigger, so it can be perceived diffraction and these allow us to hear portions diffracted despite not being in front of the sound source, or that we go through any obstacle.
The fact that they arehen appears a burden? The difference between "there is strength" and "if a charge, there would be a force" is radical. The mere possibility that at some point, a "burden of proof" to suffer a force field called Michael Faraday.
Some believe that philosophy is synonymous with wasting time. However, when the head of Michael Faraday came the concept of field, nobody could imagine how that would revolutionize science and our way of life.
Although the concept of field seems a pipe dream only convenient, Michael Faraday
insisted the reality of their existence. To Faraday, the field was a disturbance of space.
A charge produces an electric field and also occurs if you move orNo magnetic field. The
"test loads" move because of the effect of the field in which they are submerged and not due to
action at a distance from the other charge. For many, it was difficult to share the hallucinations of Faraday. The field did not seem to be something tangible, but a simple idea and assume that the ideas are part of reality, seemed a matter of ancient philosophies overcome. The reality was made things matter ... not concepts. So philosophers who sought to put both feet in reality are sometimes called, as in the expression "dialectical materialism, well known by those who have had contact with socialist literature. Electromagnetic waves
Despite its intangibility, the reality of the fields reconsultation to be as certain as the sun illuminates the earth
. When James Clerk Maxwell's equations examined
describing the interactions between electric and magnetic fields realized that predicted that a disruption
they may spread. A variation of the electric field would produce a variable magnetic field. The variation in the magnetic field is reflected in a new variation of the electric field, which in turn would cause a variation in the magnetic field. The electric and magnetic field, two disturbances of space, they propagate together in what would be an electromagnetic disturbance. The speed at which this disturbance would propagate through empty space to be strangely consultation rvery similar to the speed of light, which had been measured at the time. The conclusion was amazing, the light was nothing more than an electromagnetic wave.
already mentioned that the wavelength of light is small relative to the size of everyday objects
. The question now was: Will there be electromagnetic waves with wavelengths of other sizes? Hertz was able to generate electromagnetic waves with wavelengths much longer. These radio waves are used today to transmit radio and television, as its wavelength is large enough, can get around obstacles: we can pick up a radio signal even though we can not see station transmitting antenna. For longer or
nda slightly longer than the red color found
infrared rays, ultraviolet rays while having wavelengths slightly shorter than violet. Both infrared electromagnetic waves such as ultraviolet
have properties very similar to that of light, sometimes referred to as light. Due to this broadening of the concept, the interval between red and violet, which is what our eyes can perceive, is called visible light.
ultraviolet rays, X rays and gamma rays are just electromagnetic waves with wavelengths shorter than visible light.
X-rays, which have had a spectacular application in medicine and gamma rays that produce the elements
radioactive revealed nothing more than
electromagnetic waves of shorter wavelengths than light. Interestingly, the smaller the wavelength of an electromagnetic wave, the more likely it is harmful to living beings. It is necessary to use sunscreen to protect our skin from ultraviolet rays, X rays can not be used in pregnant women and accidents such as Chernobyl cause terrible damage due to, among other things, the gamma-ray emission.
One consequence of considering light as an electromagnetic wave is that the ether
longer necessary. The ether had played an important role in support of the theory
wave, but that role was now the concept of ccrumbs. The fields are disturbances
vacuum, and once accepted this need not conceive of a special material that can disturb electromagnetic wave
. However, the ether, like all old ideas are resisted
die. However, the successful failure of Michelson-Morley experiment, which aimed to detect the ether, and the subsequent development of the theory of relativity by Albert Einstein, killed, buried and prayed novena to the ever useful concept. But that's another story to be told another time. Wave-particle duality
far, the debate about the nature of light could be counted as if it were a soap opera. The evil with his absurd Newton corpuscular theoryr an explanation that even Maxwell's electromagnetic theory could give. One of these phenomena was the photoelectric effect.
In the photoelectric effect, light energy can be transferred to an electron depends
color. Electromagnetic theory says that the energy contained in light is related to the amplitude of the wave while the color is related to its wavelength. Nowhere electromagnetic theory proposes a relationship between energy and color. Thus, the photoelectric effect remained a mystery.
The solution to the puzzle was clever, but the physics had to take a small leap
ago. Albert Einstein said that the photoelectric effect could be explained easily ifnsferida the electron depends on the color of the light and not its intensity.
All electromagnetic waves can be understood as consisting of photons.
For very long wave lengths, the photons have very little power, so the concept is not useful. In contrast, for X rays and gamma rays, photons are highly energetic and its study is indispensable for understanding the increasingly wide variety of electromagnetic phenomena that the theory can not explain.
Despite the usefulness of the new concept, the photon seems to have revived the old corpuscular theory of Newton
. The obvious question was: Is light a wave or a particle? The
evidence in favor of electromagnetic theory do not disappearn reality, all are ondaspartículas, where they can sometimes dominate the wave properties and others, those of
particle. This concept led to the development of quantum mechanics.
say that idleness is the mother of all philosophies. The truth is that thanks to the work undertaken by the Greek philosophers, we now have a technology that frees us from repetitive work and allows us to have free time during which we can continue philosophizing.
The story is not finished being written
are resigned not all that easily. Designed experiments constantly with
purpose of determining whether the light is particle or wave back. One of these consisted of a repetition of the experiment of Young, but networkwere as powerful tools for technological development. However, we should not fall into the oft-repeated mistake of taking the latest theories known as absolute truth. The truth must be something much more complex and certainly in the future balguien venture to find a new way to explain all these phenomena, a new form, so clear and elegant, it gets to dismiss the current theories. Will this happen soon?
Will we have chance to see it? Who will be brave dare? Could it be perhaps ...
one of us?
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