In 1610, Galileo proposed that eclipses of Jupiter's moons provided a 'clock' that could be seen almost anywhere on Earth, thus providing a way to calculate longitude positions. Unfortunately, inaccuracies in Galileo's timetables made this inaccurate at first.
In 1671, from an observatory at Uranienborg (near Copenhagen), the Danish astronomers, Ole Christensen Rømer (1644 – 1710), and Jean Picard observed about 140 eclipses of Jupiter's moon, Io, over a period of several months. Meanwhile, in Paris, Giovanni Domenico Cassini observed the same eclipses and, by noting the precise difference in the times of the eclipses at both locations, they were able to calculate the time difference between Paris and Uranienborg and thus the difference in longitude.
Cassini had been observing the moons of Jupiter from 1666 to1668, and he had noticed discrepancies in his measurements that he attributed to light having a finite speed. In 1672 Rømer continued observing the satellites of Jupiter from Paris, as Cassini's assistant. Rømer observed that times between eclipses got shorter as Earth approached Jupiter, and longer as Earth moved farther away. In 1676, Cassini reported, ' . . . light seems to take between 10 and 11 minutes [to cross] a distance equal to the half-diameter of the terrestrial (Earth's) orbit.'
Rømer set out to irrefutably confirm his discovery using his observations made between 1671 and 1677 and his extensive calculations were reported on 7 December 1676. However, he did not calculate the speed of light and instead gave his data to Christiaan Huygens who derived a light speed of 212,000 km/s (kilometres per second).
Rømer's opinion that the velocity of light was finite was not fully accepted until James Bradley's measurements confirmed the idea in 1727.
In 1809, after more than a century of increasingly precise observations, Jean Baptiste Joseph Delambre reported, that light travelled from the Sun to the Earth in 8 minutes 12 seconds and the speed of light as slightly more than 300,000 km/s. (The modern value is 8 minutes 19 seconds, at a light speed of 299,792.458 km/s).
Christiaan Huygens (1629 - 1695) devised a wave theory of light in 1678 but it was not generally accepted until 1818 when Augustin-Jean Fresnel showed that it could explain the rectilinear propagation and diffraction effects of light. In 1655, Huygens designed a 50-power refracting telescope with which he discovered that the rings of Saturn were, "a thin, flat ring".
He wrote his first mathematical treatise on probability theory in 1657, with the work of Van Rekeningh, and the modern concept of probability grew out of the use of expectation values by Huygens and Blaise Pascal. In 1659, he was the first to derive the formulae for centripetal force and centrifugal force and was also the first to formulate the laws of elastic collision. The formulae played a central role in classical mechanics. Huygens is also remembered for inventing the most accurate timekeeper for the next 200 years, the pendulum clock in 1656.
In 1704, Isaac Newton estimated that it took "seven or eight minutes" for light to travel from the Sun to the Earth (the actual time is 8 minutes 19 seconds).
He also developed a sophisticated theory of colour based on the observation that a prism separates white light into the colours of the visible spectrum. He believed light was a stream of particles, although his studies of interference patterns supported the wave theory of light. This was described in his book Opticks published in 1704.
Newton's rivals, Robert Hooke and Christiaan Huyghens, believed that light was a wave but it was not until 1803 that Thomas Young, using a double-slit interferometer, demonstrated that the crests and troughs of light waves can add or subtract to give bright and dark regions. He also suggested that each colour was produced by a different wavelength and that three sets of detectors in our eyes see these primary colours. (In 1905, Einstein proposed that light did not travel in waves, as proven by many experiments, but that it travelled in 'energy quanta' (now called photons)).
In 1728, the English physicist James Bradley used the apparent changes in the position of stars caused by motion of Earth around the sun (stellar aberration) to calculate the speed of light at about 301,000 km/s.
In 1791, Michael Faraday discovered the principles of electromagnetic induction, diamagnetism and electrolysis. In 1831, he built the first electric motor and also the first electrical generator. He also demonstrated that the voltage of an alternating current could be changed by placing two coils of wire together; the voltage in one coil would create a voltage in the other in direct relationship to the number of windings in each coil. (This device is now known as an electrical transformer). The unit of capacitance, the farad, was named in his honour.
In 1849, Hippolyte Fizeau used an intense light source interrupted by a cogwheel rotating up to hundreds of times a second. The light was reflected back from a mirror about 8 km from the light. He adjusted the cogwheel speed until the returned pulses of light were blocked by the spinning cogs allowed him to estimated the speed of light to be 313,000 km/s. Léon Foucault improved this, using a rapidly rotating mirror, to get a speed of 298,000 km/s in 1862.
In 1856, Wilhelm Eduard Weber and Rudolf Kohlrausch measured the ratio of the electromagnetic and electrostatic units of charge and found the number to be very close to Fizeau's estimate of the speed of light. The following year Gustav Kirchhoff calculated that an electric signal in a resistanceless wire travelled along the wire also at the speed light.
In 1883, Albert Abraham Michelson measured the speed of light in vacuum as 299,853 ± 60 km/s. (The actual speed in a vacuum is exactly 299,792. 458 km/s).
In 1860s, James Clerk Maxwell, a Scottish scientist who studied Faraday's work, suggested that electromagnetic waves travelled at a speed near the speed of light and proposed that light was an electromagnetic wave. In 1865, he proposed that light was composed of oscillating waves in the same medium that caused electric and magnetic phenomena and that electric and magnetic fields moved as waves through space at the speed of light. In 1855, he combined all of the current knowledge into a set to four partial differential equations (now known as Maxwell's Laws).
His unification of light and electrical phenomena prompted him to predict the existence of radio waves, as well as providing the basis for quantum mechanics and inspiring Albert Einstein to formulate the theory of special relativity.
Maxwell believed that the propagation of light required a medium for the waves in the same way that water is a medium for ocean waves and air is the medium for sound waves. But, in1907, Albert Abraham Michelson and Edward Williams Morley were unable to find such a 'luminiferous aether,' and, if the absolute frame of reference necessary for an aether did not exist, there was no need to change the equations for a moving observer. This also led to the development of the Lorentz transformation.
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Atoms & Light
Non-FictionIn 1657, Otto von Guericke pumped the air from two hemispheres and eight horses could not pull them apart. Amontons found a volume of air shrank as the temperature fell and realized the it would become zero. In 1774, Joseph Priestley isolated oxyge...
