Francesco Maria Grimaldi

Italian physicist and astronomer

• Born: April 2, 1618
• Birthplace: Bologna, Papal States (now in Italy)
• Died: December 28, 1663
• Place of death: Bologna, Papal States (now in Italy)

Grimaldi is best known for his experiments with light, being the first to describe its diffraction. Light diffraction is a phenomenon that indicates that light consists of waves, and is not, as previously thought, corpuscular in nature. He also detailed and named prominent features on the Moon’s surface.

Early Life

Francesco Maria Grimaldi (frahn-CHAYS-koh mah-REE-ah gree-MAHL-dee) was the fourth son of Paride Grimaldi, a wealthy silk merchant, and his second wife, Anna Cattani. After his father’s death, Francesco and an older brother left their family to devote their lives to God as Roman Catholic priests. In March of 1632, they entered the Society of Jesus (Jesuits), a religious order devoted to helping others through education. After a three-year training course as a novitiate, Francesco took advanced training in philosophy between 1635 and 1638, consecutively attending Jesuit colleges in Parma, Ferrara, and Bologna.

From 1638 through 1642, Grimaldi taught humanities and rhetoric at the College of Santa Lucia, Bologna. From 1642 through 1645, he studied theology at the same school; additional study in philosophy earned him a doctorate in 1647. He was then appointed as a professor of philosophy, but ill-health forced him to assume a less demanding position teaching mathematics, a post he occupied for the remainder of his life. His studies in philosophy had prepared him well for teaching mathematics because natural philosophy at the time included geometry, optics, mechanics, geography, and astronomy.

Although Grimaldi was ostensibly a professor of belles lettres, his academic interests were predominantly in science. Even as a student, but particularly after beginning his teaching career at Santa Lucia, he created time to study and to engage in research in basic physics and astronomy. During the latter part of his life he devoted himself almost entirely to these subjects, eventually teaching astronomy and optics.

Life’s Work

When Grimaldi began his teaching career at the Jesuit university in Bologna, his immediate supervisor, or dean, was Giambattista Riccioli, an amateur scientist with considerable interest in physics and astronomy. Encountering a kindred spirit in Grimaldi, he enlisted his aid in scientific endeavors. In the period from 1640 through 1650, Grimaldi conducted experiments on falling bodies for Riccioli, and was able to verify that for a freely falling body vertical displacement is proportional to the square of the time the object has been falling from rest.

Commencing in 1645, Grimaldi engaged in mathematical analyses and geographic surveys, which he employed to accurately determine the meridian line for Bologna. During the course of these measurements, many made with instruments he had constructed, Grimaldi distinguished himself as both a meticulous observer and a person with consummate skill in constructing scientific instruments. These instruments included an efficient quadrant to measure the heights of lunar mountains, which were to be included in the accurate map he compiled from telescopic observations during different lunar phases. In preparing this map, Grimaldi inaugurated the procedure of naming prominent craters after illustrious philosophers, scientists, and astronomers; these names, still in use, include a crater named Grimaldi. All of his work had been incorporated into Riccioli’s Almagestum novum (1651), and he also arrayed most of the astronomical tables and measurements on fixed stars that are featured in the second volume of Riccioli’s Astronomia reformata (1665).

Although Grimaldi and Riccioli worked together as peers, Grimaldi’s most successful research on his own was in the emerging field of optics. Grimaldi’s most important discovery was that rays of light could be diffracted, or bent, when passing through a small aperture or around objects. It had been assumed earlier that light always traveled in perfectly straight lines, lending credence to the then-prevalent theory that light consisted of small rapidly moving particles called corpuscles.

Grimaldi projected a bright beam of sunlight into a darkened room through a small circular hole. The cone of light produced was projected onto a white screen at an oblique angle, thus creating an elliptical image of the sun. He then inserted a narrow opaque rod into the cone of light, casting a shadow that was considerably larger than rectilinear projection predicted. Furthermore, the shadow did not have clearly defined borders, but instead consisted of alternating dark and light bands with colored fringes. The bright band nearest the principal shadow also contained a narrow violet band close to the shadow and a narrow red band on the edge farther away.

Another experiment allowed the beam to pass through a second, somewhat larger, aperture before being projected onto the screen. The resulting spot of light was larger than the second aperture and also contained colored fringes. Grimaldi correctly concluded that the light rays had diverged slightly, becoming bent outward after passing through the second aperture. Because these phenomena cannot be explained by a corpuscular theory of light, Grimaldi deduced that light had a fluid nature that allowed it to bend around objects. He coined the word “diffraction,” based on a Latin root meaning “a breaking up,” by analogy to the manner in which a stream of water splits apart when it encounters an obstacle.

Although sound was known to be a wave phenomena that bent around obstacles analogously to the manner by which water waves diffracted around rocks, it seems that the notion that light might also consist of periodic waves never occurred to Grimaldi. He conceived of light rays as a column of fluid in vibration, but the vibration was not repetitive; it did not possess the periodicity exhibited by sound waves. To explain the varieties of color he had observed, he proposed that what has occurred was a change in the agitation of the luminous flow of light.

Grimaldi died in Bologna on December 28, 1663, after suffering from a fever for eight days. Only months before his death, he had completed his book that described his experiments. Physico-mathesis de lumine, coloribus, et iride (1665; physico-mathematical thesis on light, colors, and the rainbow; English translation, 1963) included a brief eulogy for Grimaldi by Riccioli.

Significance

Grimaldi’s most important scientific work was in optics, where his careful innovative experiments laid the foundation for the wave theory of light, a theory that was not accepted by scientists fully until the nineteenth century.

Two seventeenth century scientists greatly influenced by Grimaldi’s book were Isaac Newton , who championed the corpuscular theory of light, and the Dutch scientist Christiaan Huygens , who favored a wave theory. Huygens proposed a principle demonstrating how waves progressed through a material medium from any initial position, and he used this principle to derive the known laws of reflection and refraction. Newton, on the other hand, incorporated Grimaldi’s results along with his own careful measurements into a comprehensive corpuscular theory. Newton explained Grimaldi’s fringes as the result of attractive and repulsive forces imposed by material obstacles when light rays glazed the surface. Newton’s scientific eminence was so momentous that his viewpoint dominated throughout the eighteenth century, despite mounting evidence supporting the wave theory.

Although Grimaldi’s discovery and documentation of the diffraction of light was of fundamental importance, it was too far ahead of the known theory to be conclusive and, given Newton’s opposition to a wave theory, did not provide the impetus to advance Huygens’s ideas that it should have. The true significance of Grimaldi’s experiments was not recognized until more than a century later. He showed that the colored fringes could not be caused by reflection or refraction, but he failed to realize that they were indicative of periodic wave phenomena. Grimaldi’s diffraction experiments were posited and explained by the French physicist Augustin Fresnel in 1821.

Bibliography

Batorska, Danuta Stefania. Giovanni Francesco Grimaldi. Ph.D. dissertation, University of California, Los Angeles, 1972. An unpublished study of Grimaldi’s work.

Busacchi, V. “F. M. Grimaldi (1618-1663) e la sua opera scientifica.” In Actes du VIIe Congress International d’Histoire des Sciences. Paris: Académie Internationale d’histoire des Sciences, 1958. A survey of Grimaldi’s scientific career, including his lunar observations, mathematical and geographic measurements, and optics. In Italian.

McGrath, F. A. Grimaldi’s Fluid Theory of Light. Master’s thesis, University College, London, 1969. An unpublished but complete and effectual discussion of Grimaldi’s life and career in science.

Savelli, R. Nel terzo centenario del “De lumine” di F. M. Grimaldi. Ferrara: Università degli studi di Ferrara, 1966. Issued in honor of the tercentenary of Grimaldi’s book, this brief tract contains significant information on his life and his personality.

Sommervogel, C., ed. Bibliothèque de la Compagnie de Jésus. Nouv. éd. Mansfield, Conn.: Maurizio Martino, 1998. Grimaldi’s entire academic career is summarized in this work about the Jesuits.

Tabarroni, G. P. F. M. Grimaldi, bolognese iniziatore della ottica-fisica. Bologna, 1964. A brief work that nevertheless contains significant information on Grimaldi’s optical research.

Waldman, G. Introduction to Light. Englewood Cliffs, N.J.: Prentice Hall, 1983. An easy-to-read book that covers the nature and history of light and clearly explains optical phenomena such as diffraction.