Light Travels Slower in Water
In the 17th century, scientists began to investigate whether light has a finite and measurable speed. In 1690 Christiaan Huygens thought of light as a longitudinal wave and predicted that the wave would travel more slowly in glass or water than in air. In 1704, Isaac Newton published his theory of light as a "particle" or stream of particles. Newton's explanation of refraction – the bending of a light beam as it passes from one transparent material to another – assumed that light travels faster as it travels from air to water.
Calculations of the speed of light were based on astronomical facts showing how fast light travels through space. The first terrestrial measurement was made in 1849 by the French physicist Hippolyte Fizeau. A beam of light was held at a tooth pitch of a rotating gear wheel. Then this light was reflected by a mirror placed 8 km away and passed again through the next tooth gap of the wheel. Taking time and distance, as well as the precise speed of the spin that made this possible, Fizeau calculated the speed of light at 313,000 km/s.
Conflict with Newton
In 1850, Fizeau collaborated with his physicist friend Leon Foucault; Foucault adapted Fizeau's device and made it smaller by reflecting the beam of light from a rotating mirror instead of passing it through a cog. The light turned into the rotating mirror would be reflected on the far mirror only when the rotating mirror was at the right angle. The light returning from the fixed mirror was reflected again by the rotating mirror; but since this mirror moves as the light travels, it would not reflect directly back to the source.
In Foucault's experiment, the speed of light was measured from the angle difference created when a beam of light was reflected from a rotating mirror to the stationary mirror and back to the stationary mirror.
The speed of light could be calculated from the angle between the light traveling to and from the rotating mirror and from the rotational speed of the mirror.
The speed of light in water could be measured in the device by placing a water tube between the rotating mirror and the stationary mirror. Using this device, Foucault determined that light travels more slowly in water. According to him, light as such could not be a particle, and this experiment was seen at the time as a refutation of Newton's theory of particles. Foucault further developed his device and in 1862 measured the speed of light in air at 298,000 km/s; very close to today's 299,792 km/s.
Who is Leon Foucault?
Born in Paris, Leon Foucault was primarily homeschooled before entering medical school, where he took lessons from bacteriologist Alfred François Donne. Unable to tolerate the sight of blood, Foucault soon dropped out of school, became Donne's laboratory assistant, and found a way to take pictures with a microscope. Later, he worked with Hippolyte Fizeau and took the first photograph of the Sun.
Besides measuring the speed of light, Foucault is famous for showing experimental evidence of the Earth's rotation in 1851 using a pendulum and later a gyroscope. Although he had no formal scientific training, Foucault was given a chair at the Imperial Observatory in Paris. He also became a member of many scientific societies and is one of the 72 French scientists whose name is inscribed on the Eiffel Tower.
Historical Developments About the Speed of Light
-1676 – Ole Rømer makes the first successful measurement of the speed of light using the eclipses of Jupiter's moon Io.
-1690 - Christiaan Huygens publishes Treatise on Light, which argues that light is a type of wave.
-1704 - Isaac Newton's Opticks proposes that light is a stream of "particles".
-1864 - James Clerk Maxwell, the speed of electromagnetic waves is so close to the speed of light that he realizes that light must be an electromagnetic waveform.
-1879-83 - German-born US physicist Albert Abraham Michelson develops Foucault's method and achieves the speed of light very close to today's value.
