0.13 Chapter 4.light and matter – dielectric behaviour of matter  (Page 2/4)

Within a year of death of Galileo, Sir Issac Newton(1642-1727) was born in England. He studied the dispersion of light into seven rainbow colours (Violet, Indigo, Blue, Green, Yellow, Orange and Red) but he was unable to reconcile the rectilinear propagation of light with the spherical wave-front of light from a point source. Hence Newton favoured Corpuscular Theory of Light.

There was a problem of Chromatic Aberration in Refracting Telescope which Newton was unable to correct. So in its place he built Reflecting Telescope in 1668. This was only 6 inches long and 1 inch in diameter but it had a magnification of 30 timesa.

Christian Huygens (1629 – 1695) extended the Wave Theory of Light and gave the theoretical basis of Reflection, Refraction and Double Refraction. He discovered that there was perpendicular polarized light (perpendicular to the plane of incidence) and parallel polarized light (parallel to the plane of incidence).

Ole Christensen Romer (1644-1710) was the first person to recognize that light was not instantaneous but it had a finite velocity. Based on this reasoning in 1676, Romer predicted that on 9 ^th November Io, a moon of Jupiter, would emerge from Jupiter’s shadow 10 minutes later than what would be expected from the yearly average motion. This what happened and this led to the conclusion that light took 22 minutes to cover a distance equal the orbital diameter of Earth around Sun i.e. a distance of 2 Astronomical Unit (A.U.) where 1 A.U. = 1.49598×10 ¹¹ m. Using this data we arrive At c = 2.2664×10 ⁸ m/s but the correct value is c = 3×10 ⁸ m/s. This error occurred due to underestimation in Jupiter’s Orbit size.

Huygen and Newton based on the same reasoning arrived at c = 2.3×10 ⁸ m/s and c = 2.4×10 ⁸ m/s respectively.

Section 4.1.2. 19 th Century – the emergence of Wave Theory of Light.

Dr. Thomas Young (1773-1829) in England presented a series of papers on Wave Theory of Light in 1801, 1802 and 1803 before the Royal Society of London. He added a new dimension to the existing Wave theory by illustrating the Principle of Interference. He explained the coloured fringes in thin films and determined the wave-length of the seven colours of light.

Augustine Jean Fresnel (1788-1827) in France independently explained the diffraction pattern arising from various obstacles and apertures and he also accounted the rectilinear propagation of Light based on Wave Theory.

In 1725, James Bradley (1693-1762) attempted to measure the distance to star by triangulation method by observing a given star at six months time period. During this experiment he observed Stellar Aberration. This is different from Parallex Error.

The problem of perpendicular and parallel polarization led Young to revise the mode of propagation of Light. Initially it was assumed to be longitudinal much as the sound waves. But detection of polarization forced Young to postulate that Light had Transverse Mode of propagation.

By 1825, Wave Theory of Light was established.

In 1849, Armand Hippolyte Louis Fizeau (1819-1896) did the first terrestrial determination of the speed of light in air. It came to be 315,300 km/s.