? French engineer Augustine Fresnel (1788- 1827) was the first to correctly explain the diffraction phenomenon of light. 1806 graduated from the Polytechnic University of Paris, and then studied at the Paris Institute of Bridges and Roads for three years. After graduation, he worked as an engineer for eight years. Because of optical research, Fresnel specialized in scientific research, and began to study the interference and diffraction of light from 18 14. At that time, he didn't know the work of British scientists (1773- 1829), but like Yang, he took these phenomena as evidence of the theory of light fluctuation.
? 18 17 The French Academy of Sciences held a scientific competition, requiring participants to demonstrate all the diffraction effects of light with accurate experiments and establish corresponding theories. Fresnel decided to take part in the competition. He wrote a paper describing his research work and handed it over to the Academy of Sciences the following year. Fresnel did a series of diffraction experiments of light, and he demonstrated Grimm's discovery in more detail. Under the illumination of the light beam emitted by the light source, in the shadow of a filament, there will be a light band alternating with light. He confirmed by experiments with small holes that as long as the diameter of the hole is small enough to match the wavelength of light, the beam can produce a circular hole diffraction pattern, which is a circle of concentric circles with alternating light. He also demonstrated the diffraction effect of light passing through circular screens, sharp straight edges (such as knife edges) and slit obstacles.
? In the application paper of 18 18, Fresnel put forward a new theory called huygens-fresnel principle on the basis of introducing his diffraction experiments of various lights. Huygens thinks that the wavefront at a given time t can be regarded as the envelope of all spherical waves, and all points on the wavefront at the previous time t0 are the wave sources of these spherical waves. Fresnel's improved huygens principle. On this basis, he established his own theory. He thinks that the amplitude of light wave at a certain point in time space T can be regarded as the result of the coherence of all spherical waves, and all points on the wavefront at the previous moment t0 are the wave sources of these spherical waves.
? According to this principle, Fresnel studied various diffraction phenomena and created a mathematical method (Fresnel zone method) to quantitatively calculate the distribution of diffraction zones in these cases. He studied the light passing through a small hole and calculated what pattern should be seen on the screen behind the hole. According to his calculation, if the light is monochromatic, you should see some rings with alternating light and dark on the screen, and Fresnel calculates that the radius of these rings depends on the size of the hole, the distance from the light source to the small hole and so on.
? According to the wave theory, Fresnel also discussed other cases of light diffraction with different diffraction holes and calculated the distribution of diffraction fringes. All Fresnel calculations are consistent with the experimental results.
? The Academy of Sciences has set up a special review committee to review all applied papers. The members of the Committee were all famous French scholars at that time, namely arago (D.F. Arage, 1786- 1853) and Poisson (S.D. Poisson, 178 1-65438). 1774- 1862), Laplace (P.S. Laplace, 1749- 1827) and Gay-Lussac (J.L. Guy Lusa,1778-/. On the other hand, Gay-Lussac is neutral. Therefore, it is impossible for the Committee to trust Fresnel's research work easily, but Fresnel's calculation results are so consistent with the experimental data that Fresnel's paper cannot be denied. Finally, he had to be awarded an award. An interesting thing happened. After reading Fresnel's calculations, Poisson, a member of the appraisal committee, found that according to these calculations, an incredible conclusion would be drawn: there should be a bright spot in the shadow of a painting. But so far, no one has observed this situation, and it seems that
? Fresnel is facing this severe test. Whether his theory is correct must be tested by experiments. Fresnel's theory points out that this bright spot is obvious only when the radius of the picture is very small, so he did another experiment, and the result really got the bright spot mentioned by Poisson, which fully proved the correctness of Fresnel's wave theory. Later generations dramatically called this bright spot Poisson bright spot (or Poisson bright spot). Therefore, people often prove Fresnel's diffraction theory and its experiments.
? Fresnel's achievement is a great shock to the believers of particle theory of light, which makes Biot, Laplace, Poisson and other well-known scholars who support particle theory speechless. Encouraged by this achievement, arago changed his mind and turned to wave theory.
? During this period, J.V. Fraunhofer of Germany (1787-1826) also studied the diffraction phenomenon, but his theory was simpler than Fresnel's. Fresnel demonstrated the diffraction experiment. The light source and screen are not infinitely far away from the diffraction hole, so the diffraction hole has an opening angle. The slit is imaged near the focal plane of the eyepiece, so that the incident light and the outgoing light are parallel light. It can be considered that the distance between the light source and the light screen is infinite. The resulting diffraction is called Fraunhofer diffraction, while the former is called Fresnel diffraction. It can be seen that Fraunhofer diffraction is a limit case of Fresnel diffraction, which is easier to deal with mathematically.
? Of course, Fresnel's theory also has some shortcomings. Fresnel began to regard light waves as longitudinal waves and could not explain the polarization of light, so he tried his best to avoid these problems. It was not until Marius (E.L. Malus, 1775- 18 12) discovered the polarization of light that Fresnel regarded light as a shear wave according to Yang's proposal, which not only improved light and polarization, but also explained light and completed a series of studies on the polarization of light, which was another contribution to wave optics. Fresnel was elected as an academician of the Paris Academy of Sciences in 1823 and a member of the Royal Society of London in 1825. Unfortunately, Fresnel's successful life is too short. He only lived to be 39 years old, 1827 suffered from lung disease.
? In addition, it is worth mentioning that both Yang and Fresnel have made important contributions to wave optics, but there is no fierce dispute over priority between them, and their relationship is friendly and humble. In a letter to Yang in 18 16, Fresnel said, "If there is anything to comfort me that I didn't get priority, it is that for me, I met scholars who enriched physics with so many important discoveries. At the same time, it also greatly helps to enhance confidence in the theory I have adopted. " Yang wrote in his letter to Fresnel (18 19): "Thank you very much, Sir, for giving me an admirable paper. Among the many papers that have contributed the most to the progress of optics, your paper really has a high status. "