1665 Posthumous publication of the discovery of light diffraction by Bolognese Gesuit F. M. Grimaldi (1613-1663)

1667 First hypothesis on the wavelike nature of light by R. Hooke

1690 Traité de la lumière by C. Huygens, in which the wave theory of light is developed. But let us remember that Huygens waves do not correspond to extended wave trains but to impulses.

1801 T. Young two-slit experiment, in which we observe the interference pattern originated by the light coming from two slits, both being illuminated by the same source.

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1816-1819 A. Fresnel studies diffraction phenomena and takes part in the competition advertised by The Academy of France. He successfully evidences the presence of a light spot at the center of the shadow of a circular screen, due to the diffraction of the light wave, thus convincing a skeptical S. D. Poisson, who had proposed the test, and obtaining the prize offered.

1862-1864 J. C. Maxwell formulates the electromagnetic theory of light, according to which light consists of transversal vibrations of the same medium which originates electrical and magnetic phenomena.

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1895 W. C. Röntgen discovers X-rays

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1900 P. Villard discovers gamma-rays.

1912 W. Friedrich, P. Knipping and M. Laue observe the diffraction of X rays by a crystal , thus confirming their wavelike character.

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1914 E. Rutherford and E. N. da C. Andrade observe the diffraction, effected by a crystal, even of soft gamma rays.

1675 Corpuscular theory of light formulated by I. Newton. Actually Newton’s theory’s scope is more wide and contains also wavelike elements, since the light corpuscles are assumed to induce vibrations in the bodies. The author is aware of the results of Grimaldi’s experiment, and of course he has himself observed the Newton rings.

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1887 H. Hertz discovers the photoelectric effect while experimenting, on the way toward the verification of Maxwell’ electromagnetic theory.

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1897 G. G. Stokes hypothesizes that X-rays might be elecromagnetic impulses concentrated in time

1898-1903 J. J. Thomson accepts and developes Stoke’s hypothesis.

1899-1902 P. Lenard demonstrates that the photoelectric effect is due to the emission of electrons from the metal on which the light impinges.

1905 A. Einstein introduces the hypothesis that electromagnetic radiation is constituted by discrete light quanta having E = hv, and predicts the dependence of the kinetic energy of the electrons emitted on the frequency of the impinging radiation.

1916 A. Einstein associates to the light quantum a momentum hν/c.

1916 R. A. Millikan verifies the Einstein equation for the photoelectrice effect (but does not believe the theory of light quanta).

1921 M. De Broglie (brother of L. de Broglie) verifies the Einstein equation for the photoelectrice effect produced by X rays.

1923 A. H. Compton interprets the anelastic scattering of X rays in terms of the Einstein theory of light quanta

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1925 Bothe e Geiger demostrate the time coincidence between the emission of the photon scattered by the Compton effect and the recoil electron. Compton and Simon utilizing the cloud chamber observe that the trace of the recoil electron and that of the secondary electron produced occasionally by the scattered photon are in agreement with the principle of energy conservation and of momentum conservation. At this point the Einstein theory of light quanta is at long last accepted by most physicists.

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1926 G. N. Lewis introduces the term "photon" to designate the light quantum.

1909 G. I. Taylor observes an interference pattern using a very weak beam of light over a time of 2000 hours, in the attempt to detect a “granular” structure of light, according to the J. J. Thomson’s hypothesis of impulses.

1927 A. J. Dempster and H. F. Batho observe interference patterns produced on visible light by means of a diffraction grating when photons enter the system one by one.

1958 L. Jánossy e Zs. Náray observe interference patterns within a Michelson interferometer where photons enter one by one and each one is revealed by a photomultiplier.