Young's interference experiment
Encyclopedia
Young's interference experiment, also called Young's double-slit interferometer, was the original version of the modern double-slit experiment
, performed at the beginning of the nineteenth century by Thomas Young
. This experiment played a major role in the general acceptance of the wave theory of light. In Young's own judgment, this was the most important of his many achievements.
, Christian Huygens and Leonhard Euler
.Max Born
& Emil Wolf, Principles of Optics, 1999, Cambridge University Press However, Isaac Newton
, who did many experimental investigations of light, had rejected the wave theory of light and developed his corpuscular (or particle) theory according to which light is emitted from a luminous body in the form of tiny particles. This theory held sway until the beginning of the nineteenth century despite the fact that many phenomena, including diffraction
effects at edges or in narrow apertures, colours in thin films and insect wings, and the apparent failure of light particles to crash into one another when two light beams crossed, could not be adequately explained by the corpuscular theory which, nonetheless, had many eminent supporters, including Pierre Simon de Laplace and Jean Baptiste Biot.
where he argued that light was also a wave motion. His idea was furiously opposed because it contradicted Newton, whose views were considered sacred.
Nonetheless, he continued to develop his ideas. He believed that a wave model could much better explain many aspects of light propagation than the corpuscular model:
He demonstrated the phenomenon of interference in water waves.
In 1803, he performed his famous double-slit experiment (strictly speaking, a double hole experiment) that he later described
in a paper entitled "On the nature of light and colours":
The figure shows the geometry for a far-field viewing plane. It is seen that the relative paths of the light travelling from the two points sources to a given point in the viewing plane varies with the angle θ, so that their relative phases also vary. When the path difference is equal to an integral number of wavelengths, the two waves add together to give a maximum in the brightness, whereas when the path difference is equal to half a wavelength, or one and a half etc, then the two waves cancel, and the intensity is at a minimum. The angular spacing of the fringes, , is then given by
where <<1, and λ is the wavelength
of the light. It can be seen that the spacing of the fringes depends on the wavelength, the separation of the holes, and the distance between the slits and the observation plane, as noted by Young.
This expression applies when the light source has a single wavelength, whereas Young used sunlight, and was therefore looking at white-light fringes which he describes above. A white light fringe pattern can be considered to be made up of a set of individual fringe patterns of different colours. These all have a maximum value in the centre, but their spacing varies with wavelength, and the superimposed patterns will vary in colour, as their maxima will occur in different places. Only two or three finges can normally be observed. Young used this formula to estimate the wavelength of violet light to be 400nm, and that of red light to be about twice that - results with which we would agree today.
He presented his results to the Royal Institution, but again, he was attacked for opposing Newton's corpuscular theory.
which included Simeon Poisson were so confident that they set the subject for the next year's prize as diffraction, being certain that a particle theorist would win it. Augustin-Jean Fresnel
submitted a thesis based on wave theory and whose substance consisted of a synthesis of the Huygens' principle and Young's principle of interference.
Poisson studied Fresnel's theory in detail and of course looked for a way to prove it wrong being a supporter of the particle-theory of light. Poisson thought that he had found a flaw when he argued that a consequence of Fresnel’s theory was that there would exist an on-axis bright spot in the shadow of a circular obstacle blocking a point source
of light, where there should be complete darkness according to the particle-theory of light. Fresnel's theory could not be true, Poisson declared: surely this result was absurd. (The Poisson spot is not easily observed in every-day situations, because most everyday sources of light are not good point sources.)
However, the head of the committee, Dominique-François-Jean Arago
, and who incidentally later became Prime Minister of France, did not have the hubris of Poisson and decided it was necessary to perform the experiment in more detail. He molded a 2-mm metallic disk to a glass plate with wax. To everyone's surprise he succeeded in observing the predicted spot, which convinced most scientists of the wave-nature of light. In the end Fresnel won the competition, much to Poisson's chagrin.
After that, the corpuscular theory of light was vanquished, not to be heard of again till the 20th century. Arago later noted that the phenomenon (which was later to be
known as the Arago spot
) had already
been observed by Joseph-Nicolas Delisle
and Giacomo F. Maraldi
a century earlier.
Double-slit experiment
The double-slit experiment, sometimes called Young's experiment, is a demonstration that matter and energy can display characteristics of both waves and particles...
, performed at the beginning of the nineteenth century by Thomas Young
Thomas Young (scientist)
Thomas Young was an English polymath. He is famous for having partly deciphered Egyptian hieroglyphics before Jean-François Champollion eventually expanded on his work...
. This experiment played a major role in the general acceptance of the wave theory of light. In Young's own judgment, this was the most important of his many achievements.
Theories of light propagation in the 17th and 18th centuries
During this period, many scientists proposed a wave theory of light based on experimental observations, including Robert HookeRobert Hooke
Robert Hooke FRS was an English natural philosopher, architect and polymath.His adult life comprised three distinct periods: as a scientific inquirer lacking money; achieving great wealth and standing through his reputation for hard work and scrupulous honesty following the great fire of 1666, but...
, Christian Huygens and Leonhard Euler
Leonhard Euler
Leonhard Euler was a pioneering Swiss mathematician and physicist. He made important discoveries in fields as diverse as infinitesimal calculus and graph theory. He also introduced much of the modern mathematical terminology and notation, particularly for mathematical analysis, such as the notion...
.Max Born
Max Born
Max Born was a German-born physicist and mathematician who was instrumental in the development of quantum mechanics. He also made contributions to solid-state physics and optics and supervised the work of a number of notable physicists in the 1920s and 30s...
& Emil Wolf, Principles of Optics, 1999, Cambridge University Press However, Isaac Newton
Isaac Newton
Sir Isaac Newton PRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian, who has been "considered by many to be the greatest and most influential scientist who ever lived."...
, who did many experimental investigations of light, had rejected the wave theory of light and developed his corpuscular (or particle) theory according to which light is emitted from a luminous body in the form of tiny particles. This theory held sway until the beginning of the nineteenth century despite the fact that many phenomena, including diffraction
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...
effects at edges or in narrow apertures, colours in thin films and insect wings, and the apparent failure of light particles to crash into one another when two light beams crossed, could not be adequately explained by the corpuscular theory which, nonetheless, had many eminent supporters, including Pierre Simon de Laplace and Jean Baptiste Biot.
Youngs' work on wave theory
While studying medicine at Gottingen in the 1790s, Young wrote a thesis on the physical and mathematical properties of sound and in 1799, he presented a paper to the Royal SocietyRoyal Society
The Royal Society of London for Improving Natural Knowledge, known simply as the Royal Society, is a learned society for science, and is possibly the oldest such society in existence. Founded in November 1660, it was granted a Royal Charter by King Charles II as the "Royal Society of London"...
where he argued that light was also a wave motion. His idea was furiously opposed because it contradicted Newton, whose views were considered sacred.
Nonetheless, he continued to develop his ideas. He believed that a wave model could much better explain many aspects of light propagation than the corpuscular model:
In 1803, he performed his famous double-slit experiment (strictly speaking, a double hole experiment) that he later described
in a paper entitled "On the nature of light and colours":
The figure shows the geometry for a far-field viewing plane. It is seen that the relative paths of the light travelling from the two points sources to a given point in the viewing plane varies with the angle θ, so that their relative phases also vary. When the path difference is equal to an integral number of wavelengths, the two waves add together to give a maximum in the brightness, whereas when the path difference is equal to half a wavelength, or one and a half etc, then the two waves cancel, and the intensity is at a minimum. The angular spacing of the fringes, , is then given by
where <<1, and λ is the wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of the light. It can be seen that the spacing of the fringes depends on the wavelength, the separation of the holes, and the distance between the slits and the observation plane, as noted by Young.
This expression applies when the light source has a single wavelength, whereas Young used sunlight, and was therefore looking at white-light fringes which he describes above. A white light fringe pattern can be considered to be made up of a set of individual fringe patterns of different colours. These all have a maximum value in the centre, but their spacing varies with wavelength, and the superimposed patterns will vary in colour, as their maxima will occur in different places. Only two or three finges can normally be observed. Young used this formula to estimate the wavelength of violet light to be 400nm, and that of red light to be about twice that - results with which we would agree today.
He presented his results to the Royal Institution, but again, he was attacked for opposing Newton's corpuscular theory.
Acceptance of the wave theory of light
In 1817, the corpuscular theorists at the Academie FrancaiseAcadémie française
L'Académie française , also called the French Academy, is the pre-eminent French learned body on matters pertaining to the French language. The Académie was officially established in 1635 by Cardinal Richelieu, the chief minister to King Louis XIII. Suppressed in 1793 during the French Revolution,...
which included Simeon Poisson were so confident that they set the subject for the next year's prize as diffraction, being certain that a particle theorist would win it. Augustin-Jean Fresnel
Augustin-Jean Fresnel
Augustin-Jean Fresnel , was a French engineer who contributed significantly to the establishment of the theory of wave optics. Fresnel studied the behaviour of light both theoretically and experimentally....
submitted a thesis based on wave theory and whose substance consisted of a synthesis of the Huygens' principle and Young's principle of interference.
Poisson studied Fresnel's theory in detail and of course looked for a way to prove it wrong being a supporter of the particle-theory of light. Poisson thought that he had found a flaw when he argued that a consequence of Fresnel’s theory was that there would exist an on-axis bright spot in the shadow of a circular obstacle blocking a point source
Point source
A point source is a localised, relatively small source of something.Point source may also refer to:*Point source , a localised source of pollution**Point source water pollution, water pollution with a localized source...
of light, where there should be complete darkness according to the particle-theory of light. Fresnel's theory could not be true, Poisson declared: surely this result was absurd. (The Poisson spot is not easily observed in every-day situations, because most everyday sources of light are not good point sources.)
However, the head of the committee, Dominique-François-Jean Arago
François Arago
François Jean Dominique Arago , known simply as François Arago , was a French mathematician, physicist, astronomer and politician.-Early life and work:...
, and who incidentally later became Prime Minister of France, did not have the hubris of Poisson and decided it was necessary to perform the experiment in more detail. He molded a 2-mm metallic disk to a glass plate with wax. To everyone's surprise he succeeded in observing the predicted spot, which convinced most scientists of the wave-nature of light. In the end Fresnel won the competition, much to Poisson's chagrin.
After that, the corpuscular theory of light was vanquished, not to be heard of again till the 20th century. Arago later noted that the phenomenon (which was later to be
known as the Arago spot
Arago spot
In optics, an Arago spot, Fresnel bright spot, or Poisson spot is a bright point that appears at the center of a circular object's shadow due to Fresnel diffraction...
) had already
been observed by Joseph-Nicolas Delisle
Joseph-Nicolas Delisle
Joseph-Nicolas Delisle was a French astronomer.-Life:He was one of the 11 sons of Claude Delisle . Like many of his brothers, among them Guillaume Delisle, he initially followed classical studies. Soon however, he moved to astronomy under the supervision of Joseph Lieutaud and Jacques Cassini...
and Giacomo F. Maraldi
Giacomo F. Maraldi
Giacomo Filippo Maraldi was an French-Italian astronomer and mathematician. His name is also given as Jacques Philippe Maraldi....
a century earlier.
See also
- Corpuscular theory of light
- Photoelectric effectPhotoelectric effectIn the photoelectric effect, electrons are emitted from matter as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as photoelectrons...
- Wave–particle dualityWave–particle dualityWave–particle duality postulates that all particles exhibit both wave and particle properties. A central concept of quantum mechanics, this duality addresses the inability of classical concepts like "particle" and "wave" to fully describe the behavior of quantum-scale objects...