In the aftermath of the American and French revolutions, the science of optics experienced its own revolution during the early nineteenth century. Not even a century after the publication of Opticks, English physician and physicist Thomas Young challenged Isaac Newton's corpuscular theory of light. In 1801, Young did an experiment that established the principle of light interference, which could not be explained by a particle theory of light. His experiment allowed light to pass through two closely set pinholes onto a screen where he observed that the beams spread out, or diffracted, and overlapped. In areas where the light beams overlapped, bands of brightness alternated with bands of darkness.

This phenomenon was called interference and Young compared it to waves in water, where the crests of waves meet and combine to make bigger waves or crests and troughs meet and cancel each other out. In 1817, he concluded that they propagate as transverse waves, not longitudinal waves as he originally proposed. Although Young's theory was greeted with a great deal of skepticism in England, two French physicists, Augustin-Jean Fresnel and François Arago, confirmed his wave theory through their own experiments and Fresnel's detailed mathematical analysis.

An accidental discovery in 1808 also provided more evidence for the wave theory. Étienne-Louis Malus, a French engineer, was in his Paris apartment one day toying with a piece of Iceland spar, a crystal known for its property of double refraction; anything viewed through it appeared as two images. Malus was peering through the crystal at an image of the sun that had been reflected from a window across the street. Strangely, the crystal displayed only one image, not the two that Malus was expecting to see. When light reflected off a surface, apparently some of the light was filtered, or polarized. As it turned out, the theory of light as transverse waves explained this phenomenon better than any other theory.

Another accidental discovery hinted at a relationship between electricity and magnetism, and would seriously impact the theory of light several decades later. In 1820, Hans Christian Ørsted noticed that a wire carrying an electric current caused a magnetized compass needle nearby to move, aligning itself perpendicularly to a current-carrying wire. (Although he is credited with discovering this relationship, an Italian, Gian Domenico Romagnosi, had already made the discovery in 1802, but was ignored when he announced his findings.) In 1831, Michael Faraday observed the opposite effect, that a magnet moving through a coil of conductive wire generated an electric current.

Newton's color spectrum went through its own revolution early in the century. In 1802, William Hyde Wollaston discovered seven bands of darkness interrupting what had been presumed to be a continuous range of colors in the solar spectrum. Ten years later, Joseph von Fraunhofer found and measured the positions of over 300 dark lines in the spectrum, establishing the groundwork for a new field of inquiry--spectroscopy.

At about the same time, William Herschel and Johann W. Ritter discovered that there are regions of the spectrum invisible to the human eye. In 1800, Herschel was studying the relationship between light and heat. Using a prism and thermometer with blackened bulbs (for better heat absorption), he measured the temperature of each color of the sun's spectrum. As a control, he measured the temperature just outside the visible spectrum and, to his surprise, found that a region beyond the red portion of the spectrum had the highest temperature. He had discovered an area of the spectrum that could be measured and felt, but not seen--the infrared region.

A year later, Ritter discovered that the other end of the solar spectrum extends beyond the visible. He observed that silver chloride darkened when exposed to visible sunlight, but darkened even more when exposed to invisible radiation beyond the violet end of the spectrum--the ultraviolet region.

1800 - 1833 1800 William Herschel, a German-born British astronomer, discovers the infrared region of sunlight. This is the first observation of a form of light that is invisible to the human eye. 1801 Thomas Young, an English physician and physicist, discovers light interference, establishing that light is a wave and challenging Isaac Newton's corpuscular theory of light. 1801 Physicist Johann Wilhelm Ritter (Germany) finds that the sun emits invisible ultraviolet radiation. His discovery expands the sun's spectrum beyond the violet range of the visible light spectrum. 1802 William Hyde Wollaston (England) discovers that the sun's spectrum is not a continuous gradient but is interrupted by a number of dark lines. 1807 William Hyde Wollaston invents the camera lucida, a four-sided prism mounted on a small stand above a sheet of paper, which makes it possible to sketch objects more accurately. 1808 Étienne-Louis Malus (France) discovers that reflected sunlight is plane-polarized. 1811 Two French physicists, Augustin-Jean Fresnel and François Arago discover that two beams of light, polarized in perpendicular directions, do not interfere. 1812-1814 Joseph von Fraunhofer (Germany) measures the positions of 324 of the 500 or so dark spectral lines, first seen by Wollaston, setting the stage for the development of spectroscopy. 1815 David Brewster (Scotland) describes a simple mathematical relationship between the refractive index of a reflective substance and the angle at which light striking the substance will be polarized. 1816 Augustin-Jean Fresnel (France), presents a rigorous mathematical treatment of diffraction and interference phenomena, successfully explaining them with wave theory. The corpuscular theory is all but dead. 1817 Thomas Young proposes that light waves are transverse not longitudinal. They vibrate perpendicular to the direction of travel, not in the direction of travel, as do sound waves. 1819 Siméon-Denis Poisson (France) objects to Fresnel's mathematical theory of diffraction. The Paris Académie calls for an experiment, which proves Fresnel's theory to be correct. Fresnel's paper wins a prize that had been offered for a memoir on diffraction. 1820 Hans Christian Ørsted (Denmark) observes that electric current in a wire can deflect a magnetized compass needle. 1821 Joseph von Fraunhofer builds the first diffraction grating, composed of 260 very closely spaced parallel wires. 1821 Augustin-Jean Fresnel (France) presents the laws that will enable scientists to calculate the intensity and polarization of reflected and refracted light. 1826 Joseph-Nicéphore Niepce (France) uses a camera obscura to expose an emulsion consisting of Bitumen of Judea (a form of asphalt) on a pewter plate, and successfully produces the earliest known photograph. 1827 The first achromatic microscope lens system is introduced by Giovanni Baptiste Amici (Italy). 1828 William Nicol (Scotland) invents the "nicol prism," a polarizing prism made of two calcite components. 1831 Michael Faraday (England) discovers that moving a permanent magnet in and out of a coil of conducting wire can induce an electric current. From this he formulates a law of induction. 1833 A French optician, Camille Sébastien Nachet, introduces one of the first microscopes to feature crossed-polarized illumination for the examination of birefringent samples.

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