Francesco Maria Grimaldi showed that light passing through a small opening cannot be prevented from slightly spreading on the farther side. He termed this phenomenon “diffraction” and postulated that it was caused by light having a fluid nature analogous to a flowing stream of water.
In about 1655, while serving as a mathematics instructor at the Jesuit University of Santa Lucia in Bologna, Francesco Maria Grimaldi began an elaborate set of optical experiments that occupied him for the remainder of his life. These experiments clearly demonstrated that light propagating through air does not simply travel in straight lines, but tends to bend slightly around objects. This new phenomenon Grimaldi termed “diffraction” (from the Latin, “a breaking up”) because it indicated that light has a fluid nature allowing it to flow around objects like a stream of water divides around a slender obstacle in its path.
Prior to Grimaldi’s experiments scientists assumed that light always propagates rectilinearly if it remains in the same medium, which gave credence to the prevailing view that light consists of small, rapidly moving particles. It was known since antiquity that when light enters a different medium, for example, from air to water, it is bent, or refracted. Diffraction is a bending of light around objects or through openings in the same medium. Diffraction is exhibited by all types of waves—water, sound, and light—but had not been observed previously for light because the extremely small wavelengths render the effects difficult to perceive. Grimaldi’s experiments on diffraction were of two different types: One type examined the shadows produced by opaque objects of different shapes, the other type examined light passing through circular apertures.
For the shadow experiments, Grimaldi allowed bright sunlight to enter a darkened room through a tiny hole (1/60 of an inch in diameter). This created a cone of light that Grimaldi projected on a white screen set obliquely to form an elliptical image of the sun. Between the hole and the screen he inserted a narrow opaque rod to create a shadow. Examining this shadow carefully, Grimaldi observed that its size was somewhat smaller than the linear projection of light rays predicted and, even more surprising, that the shadow’s border was bounded by narrow fringes of color. He described these diffraction bands in some detail; there are usually three and they increase in intensity and width nearer to the shadow. The closest band consists of a central white region flanked by a narrow violet band near the shadow and a slender red band away from the shadow. Grimaldi cautioned that these color bands must be observed carefully to avoid mistaking the series for alternating stripes of light and dark.
Next, he examined the effect of varying the shape of the opaque object by replacing the rod with a step-shaped object with two rectangular corners. He meticulously recorded how the bands curved around the outer corner and continued to follow the shadow’s edge. He also described that when the two series of bands from each edge of the inner corner approach they intersect perpendicularly to create regions of brighter color separated by darker areas.
Grimaldi also employed several L-shaped objects of different width to study the color bands produced. His diagrams show two sets of continuous tracks, parallel to the borders, which connect by bending around in a semicircle at the end of the L. He noted that the bands appear only in pairs, the number increasing with the width of the obstacle and its distance form the screen. He also observed that at the corners of the L, an additional series of shorter and brighter colors emerged. He diagramed these as five feather-shaped fringes radiating from the corner and crossing the paired tracks of light perpendicularly. Grimaldi compared this to the wash behind a moving ship.
Grimaldi’s aperture experiment allowed the cone of light to pass through a second hole, about 1/10 inch in diameter, before being projected on a wall. The distances between the holes and between the wall and the second hole were equal at about 12 feet. Grimaldi observed that the circle of light cast on the opposite wall was slightly larger than predicted by rectilinear propagation theory, and the border displayed the same red and blue bands. He also mentioned that these diffraction effects are quite small and only observable if extremely small apertures are used.
Grimaldi also discovered that when sunlight entered a room through two small adjacent apertures, the region illuminated by the two beams was darker than when illuminated by either aperture separately. Although he did not understand that he was observing the now well-known principle of “interference of light waves,” he regarded it as conclusive proof that light was not a material, particulate substance.
Grimaldi’s carefully executed experiments convinced him that light had a liquid nature, a column of pulsating fluid that could produce color fringes when the luminous flow was agitated. The colors were inherent in the white light itself and not created by some outside agent. Although the diffraction effect so carefully measured and documented by Grimaldi is an unequivocal indicator that light consists of periodic waves, this notion seems not to have occurred to him.
Grimaldi detailed his experiments on diffraction, along with many other optical topics, in his comprehensive treatise Physico-mathesis de lumine, coloribus, et iride
Encouraged by Francesco Maria Grimaldi’s work, Christiaan Huygens
Isaac Newton,
The issue was finally resolved in favor of the wave theory by English scientist Thomas Young (1773-1829), when, in 1802, he published experimental results documenting light interference and proving that Newton’s experiments were easily explained by the wave theory. The final nail in the coffin lid of the particle theory was the experimental measurement of the speed of light underwater, accomplished in 1850 by the French physicist Leon Foucault (1819-1868). His precise measurements proved that the speed of light under water was considerably less than its speed in air, as predicted by the wave theory.
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McGrath, F. A. Grimaldi’s Fluid Theory of Light. Unpublished master’s thesis. University College, London, 1969. A complete and effectual discussion of Grimaldi’s life and career in science, with emphasis on his optical research. -
Pedrotti, L., and F. Pedrotti. Optics and Vision. Upper Saddle River, N.J.: Prentice Hall, 1998. This text presents optical principles, including an entire chapter devoted to diffraction, in an easy-to-read manner with a minimum of mathematics. The introductory chapter offers a short but complete history of the sciences of vision and of light. -
Waldman, G. Introduction to Light. Englewood Cliffs, N.J.: Prentice Hall, 1983. This book covers the nature and history of light and clearly explains optical phenomena such as diffraction.
Galileo; Francesco Maria Grimaldi; Christiaan Huygens; Sir Isaac Newton.