In 1676, Danish astronomer Ole Rømer estimated of the speed of light by observing the periods of Jupiter's innermost moon, Io. These appeared longer when the Earth was receding from Jupiter than when it was approaching. He found that light took 22 minutes to cross the diameter of Earth's orbit. Christiaan Huygens used this, and an estimate of the diameter of the Earth's orbit, to calculate the speed of light as 220,000 km/s (kilometres per second).
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).
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 1849, Hippolyte Fizeau used an intense light source interrupted by a rotating cogwheel with 720 notches that could be rotated at a variable speed of up to hundreds of times a second. The light was reflected back from a mirror about 8 km from the light. Adjusting 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. (The actual speed in a vacuum is exactly 299,792. 458 km/s).
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 the 1860s, James Clerk Maxwell suggested that electromagnetic waves travelled at a speed near the speed of light estimated by Weber, Kohlrausch and Fizeau and proposed that light was in fact an electromagnetic wave.
In 1883, Albert Abraham Michelson measured the speed of light in vacuum as 299,853 ± 60 km/s. In 1907 he conducted an experiment with Edward Williams Morley in a fruitless attempt to detect the existence of the hypothesized aether.
The radio spectrum is the part of the electromagnetic spectrum with frequencies from 30 Hz (hertz = cycles per second) to 300 Ghz (gigahertz). It is used for radio transmissions including cell phones.
Microwaves are radio waves of with frequencies between 3 and 300 Ghz used for most modern radars, radio astronomy, microwave devices, microwave radio relay, microwave remote sensing, wireless communications, local area computer networks (LAN), dedicated short-range communication (DSRC), satellite television and radio broadcasting, radio communication satellites, directed-energy weapons, cable and amateur radio.
The infrared part of the electromagnetic spectrum covers the range from about 300 GHz to 400 THz (1 Teraherz = 1000 GHz)) (wavelengths between 1 mm and 750 nm (nanometres)).
The far-infrared is the range 300 GHz to 30 THz (wavelengths 1 mm – 10 μm (micrometres)). The lower part of this range is strongly absorbed by water in the Earth's atmosphere.
The mid-infrared ranges from 30 to 120 THz (wavelengths 10–2.5 μm (micrometres)).
Hot objects (black-body radiators) including human skin, radiate strongly at the lower end of this range. The near-infrared ranges from 120 to 400 THz (2,500–750 nm (nanometres)). The highest frequencies in this range can be detected directly by image sensors for infrared photography and by some photographic film.
Optical fibre transmits light (usually in the infrared range) that can carry information. This is modulated in a way similar to that used with radio waves.
YOU ARE READING
Invention
Non-FictionIn 1885 a Pennsylvanian oil field produced 77 % of the world's oil supply. By 1889, J.Maxwell's equations for electro-magnetism and H.R. Hertz's, proof inspired Einstein's theory of relativity. By 1841, W.Talbot and J. Herschel had fixed photographi...
