Augustin Jean Fresnel: The Pioneer of Wave Optics
Augustin Jean Fresnel was a French physicist and engineer who made significant contributions to the field of optics. He was one of the first to establish the wave theory of light, which explained phenomena such as interference, diffraction and polarization. He also invented the Fresnel lens, a type of lens that is widely used in lighthouses, projectors and magnifiers.
Early Life and Education
Fresnel was born on May 10, 1788, in Broglie, France. His father was a landowner and his mother was the niece of the painter Jean-HonorÃ© Fragonard. Fresnel had a brother, Fulgence, who also became a physicist and an uncle, LÃ©onor MÃ©rimÃ©e, who was a writer and a friend of Voltaire. Fresnel showed an interest in mathematics and science from an early age. He attended the Ãcole Centrale in Caen and then the Ãcole Polytechnique in Paris, where he studied engineering. He graduated in 1806 and joined the Corps des Ponts et ChaussÃ©es (Corps of Bridges and Roads), a government agency responsible for public works.
Career and Research
Fresnel worked as an engineer in various regions of France, building roads, bridges and canals. He also pursued his passion for optics in his spare time. He was influenced by the work of Thomas Young, an English physicist who demonstrated the interference of light waves in 1801. Fresnel conducted his own experiments with devices that produced interference fringes and diffraction patterns. He used the principle of Christiaan Huygens, a Dutch scientist who proposed that every point on a wavefront acts as a secondary source of spherical wavelets. Fresnel developed mathematical formulas to describe the behavior of light waves under different conditions.
In 1814, Fresnel wrote a memoir on diffraction that he submitted to the French Academy of Sciences. However, his work was rejected by the majority of the members, who favored Isaac Newton’s corpuscular theory of light. The corpuscular theory assumed that light consists of tiny particles that travel in straight lines. It could not explain phenomena such as interference and diffraction, which implied that light behaves like a wave.
In 1815, Fresnel lost his job temporarily during the political turmoil following Napoleon’s return from Elba. He used this opportunity to continue his research on optics. He collaborated with FranÃ§ois Arago, another French physicist who supported the wave theory of light. They studied the laws of interference of polarized light, which is light that vibrates in a single plane. They also discovered circularly polarized light, which is light that rotates around its direction of propagation.
In 1819, Fresnel entered another memoir on diffraction to a competition organized by the Academy of Sciences. This time, he won the prize, thanks to an unexpected observation made by one of the judges, SimÃ©on-Denis Poisson. Poisson pointed out that Fresnel’s theory predicted that there would be a bright spot at the center of the shadow cast by a small spherical obstacle. This seemed absurd to Poisson, but Arago performed an experiment that confirmed Fresnel’s prediction. This spot became known as Poisson’s spot or Arago’s spot.
Fresnel also worked on the problem of refraction and reflection of light at the interface between two transparent media. He derived equations that related the angles and intensities of the incident, reflected and refracted rays to the indices of refraction of the media. These equations are known as Fresnel’s equations or Fresnel’s laws.
Another topic that Fresnel investigated was the phenomenon of double refraction or birefringence. This occurs when light passes through certain crystals, such as calcite or quartz, and splits into two rays with different directions and polarizations. Fresnel generalized Huygens’ theory of double refraction for uniaxial crystals (those with one optical axis) to biaxial crystals (those with two optical axes). He also explained how to determine the optical axes and indices of refraction of biaxial crystals using a device called a Fresnel rhomb.