Nuclear

Part 6 - Light

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.

Isaac 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, correctly, that each colour was produced by a different wavelength and that three sets of detectors in our eyes see these primary colours. Young's theories, subsequently confirmed by Augustin-Jean Fresnel, influenced William Herschel, James Clerk Maxwell and Albert Einstein.

In 1856, Wilhelm Eduard Weber and Rudolf Kohlrausch found that the ratio of the electromagnetic and electrostatic units of charge was close to the estimated speed of light. In 1864, Weber studied magnetism with Carl Friedrich Gauss.

The following year Gustav Kirchhoff calculated that an electric signal, in a theoretically resistanceless wire, travelled along the wire at the speed light.

Between 1848 and 1849 Hippolyte Fizeau determined the speed of light to be 315,000 km/s between an intense light source reflected from a mirror about 8 km from the source. The light source was interrupted by a rotating cogwheel with 720 notches that could be rotated at a variable speed of up to hundreds of times a second. 

By 1862, Léon Foucault substituted a rotating mirror for Fizeau's toothed wheel and measured the speed of light to be 298,000 km/s (within 0.6% of the modern value).

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).

Michael Faraday had discovered the principles of electromagnetic induction, diamagnetism and electrolysis in 1791 and by 1831 he had built the first electric motor, the first electrical generator and the first electrical transformer. 

In the 1860s, James Clerk Maxwell studied Faraday's work and proposed that light was electro-magnetic radiation composed of waves that moved 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 caused him to predict the existence of radio waves (also electro-magnetic radiation), as well as providing the basis for quantum mechanics and inspiring Albert Einstein to formulate the theory of special relativity. 

In 1887, Heinrich Rudolf Hertz, conclusively proved the existence of electromagnetic waves predicted by James Clerk Maxwell's equations of electromagnetism. The unit of frequency, one cycle per second, was named the "Hertz" in his honour. 

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, in1887, 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 concept led to the development of the Lorentz transformation. 

Meanwhile, in 1845, James Prescott Joule, an English physicist and brewer, published his discovery of the relationship of heat and mechanical work with a measurement of 772.692 ft·lbf/Btu (foot-pound force per British thermal unit) equivalent to 4.159 J/cal (Joule per calorie), close to 20th century estimates. Joule's work led to the first law of thermodynamics and the law of conservation of energy (energy can be transformed from one form to another, but can be neither be created nor destroyed).