Starting with Roemer's 1676 breakthrough endeavors, the speed of light has been measured at least 163 times utilizing a wide variety of different techniques by more than 100 investigators (see Table 1 for a compilation of methods, investigators, and dates). As scientific methods and devices were refined, the error limits of the estimates narrowed, although the speed of light has not significantly changed since Roemer's seventeenth century calculations. Finally in 1983, more than 300 years after the first serious measurement attempt, the speed of light was defined as being 299,792.458 kilometers per second by the Seventeenth General Congress on Weights and Measures. Thus, the meter is defined as the distance light travels through a vacuum during a time interval of 1/299,792,458 seconds. In general, however, (even in many scientific calculations) the speed of light is rounded to 300,000 kilometers (or 186,000 miles) per second. Arriving at a standard value for the speed of light was important for establishing an international system of units that would enable scientists from around the world to compare their data and calculations.

Measuring the Speed of Light - Light traveling in a uniform substance, or medium, propagates in a straight line at a relatively constant speed, unless it is refracted, reflected, diffracted, or perturbed in some other manner. This well-established scientific fact is not a product of the Atomic Age or even the Renaissance, but was originally promoted by the ancient Greek scholar, Euclid, somewhere around 350 BC in his landmark treatise Optica. By the late 1960s, lasers were becoming stable research tools with highly defined frequencies and wavelengths. It quickly became obvious that a simultaneous measurement of frequency and wavelength would yield a very accurate value for the speed of light, similar to an experimental approach carried out by Keith Davy Froome using microwaves in 1958. Several research groups in the United States and in other countries measured the frequency of the 633-nanometer line from an iodine-stabilized helium-neon laser and obtained highly accurate results. In 1972, the National Institute of Standards and Technology employed the laser technology to measure the speed at 299,792,458 meters per second (186,282 miles per second), which ultimately resulted in the redefinition of the meter through a highly accurate estimate for the speed of light.

Alhazen (965-1040) - Born in Iraq as Abu Ali Hasan Ibn al-Haitham, the great Arab physicist is more often known by the Latinized version of his first name, Alhazen. The efforts of Alhazen resulted in over one hundred works, the most famous of which was "Kitab-al-Manadhirn", rendered into Latin in the Middle Ages. The translation of the book on optics exerted a great influence upon the science of the western world, most notably on the work of Roger Bacon and Johannes Kepler. A significant observation in the work contradicted the beliefs of many great scientists, such as Ptolemy and Euclid. Alhazen correctly proposed that the eyes passively receive light reflected from objects, rather than emanating light rays themselves.

Dominique-François-Jean Arago (1786-1853) - In 1811, Arago, in collaboration with Augustin-Jean Fresnel, discovered that two beams of light polarized in perpendicular directions do not interfere, eventually resulting in the development of a transverse theory of light waves. Arago was also instrumental in the success and funding of Louis-Jacques-Mandé Daguerre's photographic process, known as the daguerreotype, and directed studies that directly led to the discovery of the location of Neptune by Urbain-Jean-Joseph Le Verrier.

Roger Bacon (1214-1294) - Roger Bacon was an English scholastic philosopher who was also considered a scientist because he insisted on observing things for himself instead of depending on what other people had written. Bacon's writings included treatises on optics (then called perspective), mathematics, chemistry, arithmetic, astronomy, the tides, and the reformation of the calendar. His skill in the use of optical and mechanical instruments caused him to be regarded by many as a sorcerer. Bacon was acquainted with the properties of mirrors, knew the powers of steam and gunpowder, had a working knowledge in microscopy, and possessed an instrument very much like a modern telescope.

Albert Einstein (1879-1955) - Albert Einstein was one of the greatest and most famous scientific minds of the 20th century. The eminent physicist is best remembered for his theories of relativity, as well as his revolutionary notion concerning the nature of light. However, his innovative ideas were often misunderstood and he was frequently ridiculed for his vocal involvement in politics and social issues. The birth of the Manhattan Project yielded an inexorable connection between Einstein's name and the atomic age. However, Einstein did not take part in any of the atomic research, instead preferring to concentrate on ways that the use of bombs might be avoided in the future, such as the formation of a world government.

Armand Fizeau (1819-1896) - Armand Fizeau is best known for being the first to develop a reliable experimental method of determining the speed of light on the Earth. Previously, the speed of light was measured based upon astronomical phenomena. Fizeau also conducted experiments that demonstrated that the velocity of light is a constant, regardless of the motion of the medium it is passing through. It was previously established that light traveled at different rates through different mediums, but prior to Fizeau's discovery, it was believed that if the medium was in motion, the velocity of the speed of light would be increased by the movement of the medium.

Jean-Bernard-Leon Foucault (1819-1868) - Jean-Bernard-Leon Foucault was a French physicist who is considered one of the most versatile experimentalists of the nineteenth century. Together with the French physicist Armand Fizeau, Foucault developed a way to measure the speed of light with extreme accuracy. He also proved independently that the speed of light in air is greater than it is in water. Foucault's other contributions to the field of optics included a method of measuring the curvature of telescope mirrors, an improved technique to silver astronomical mirrors, a method of testing telescope mirrors for surface defects, and the invention of a polarizing prism to analyze polarized light.

Johannes Kepler (1571-1630) - Johannes Kepler was a sixteenth century German astronomer and student of optics who first delineated many theories of modern optics. In 1609, he published "Astronomia Nova" delineating his discoveries, which are now called Kepler's first two laws of planetary motion. This work established Kepler as the "father of modern science", documenting how, for the first time, a scientist dealt with a multitude of imperfect data to arrive at a fundamental law of nature.

Albert Michelson (1852-1931) - Albert Abraham Michelson, a Polish-American physicist, was awarded the Nobel Prize in Physics in 1907. He is best known for his experiments in which he proved that the hypothetical medium of light, the "ether", did not exist, and his many attempts at accurately measuring the speed of light. Michelson is also well known for developing a means to more accurately measure the speed of light and the size of stars.

Ole Christensen Roemer (1644-1710) - Roemer' s greatest achievement was the first relatively accurate measurement of the speed of light, a feat he accomplished in 1676. At the Royal Observatory in England, Roemer's studies of Jupiter's moon Io and its frequent eclipses enabled him to predict the periodicity of an eclipse period for the moon. By applying the relatively inaccurate calculations for the distances between Earth and Jupiter available during the seventeenth century, Roemer was able to approximate the speed of light to be 137,000 miles (or 220,000 kilometers) per second.

Speed of Light in Transparent Materials - When light traveling in a vacuum enters a new transparent medium, such as air, water, or glass, the speed is reduced in proportion to the refractive index of the new material. This interactive tutorial explores the reduction in the speed of light as a function of refractive index in common substances.

Reference Listing - Understanding and measuring the speed of light has challenged the minds of the great physicists from around the world over the ages. Even in today's technologically rich society, new discoveries still produce scientific debate and advances. Included in this section are references to books and review articles that discuss theoretical and applied aspects relating to the measurement of the speed of light, and how this constant effects our understanding of the physics of light and color.

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