Optics Series Lecture,
“Electromagnetic Nature of Light — A brief history of light”
This lecture was delivered on 16th March, yesterday, in a lecture session of 1 and 1/2 hours. This lecture was delivered to Physics honors as well as Physics elective students.
As I promised in the last lecture, lecture-X we have our one of the interesting historical and technical perspective about light that is also one of my favorite, as I discovered yesterday, or shortly before that, the night before, when I was composing the lecture from scratch. We will name this lecture with its proper number, only after its clear to us what chronological number must be associated with it. Its like an advanced wave, it reached us before in time, before it was intended to be taken up for its web-version.
Let us begin this lecture which has roughly two parts, 1. the history of light and its understanding through the centuries and 2. the electromagnetic nature of light. The second part is intended as the course material for honors as well as elective students but you will be in amusement if you also cover the first part.
A brief history of light.
Various optical devices and optical phenomena have been known since close to 4000 years. The optical devices of ancient time includes mirrors, burning glasses, lenses and other magnifying devices.
Accordingly various properties and laws of light were understood and developed since these times. Eg light was understood to propagate rectilinearly, light was understood to reflect and refract. There were various laws that were known since these times which catered to the need for explanation of these phenomena. eg Reflection was understood to be a phenomena explained by the principle of shortest path — follow link to know this and other related ideas and their history: Hero of Alexandria. Laws of refraction were understood either partially or completely as the centuries or even millennia passed.
Apart from rectilinear propagation of light it was understood that light moves at infinitely large speed. Advanced optical devices such as telescopes were developed based on partial and faulty understanding of light which was gradually refined to accommodate better credits of advancement. Eyes as optical devices were understood and eye defects could be corrected by using suitable optical devices such as eye-glasses. Albeit all this light was never understood properly before the 17th CE.
The last 4 centuries saw tremendous leaps of understanding and applications of light. In the 17th CE great progress were made to understand various phenomena exhibited by light such as reflection, refraction and total internal reflection etc. Descartes proposed light as a longitudinal pressure vibration in elastic medium. Human beings understand by way of imitation and this was the reason light as waves were considered exactly in the image of sound as waves. The mantra lies in keeping the mind open for successive refinements through acceptance of truth as ordained eg by experimental facts.
Thus light was considered to be a wave. During 17th CE, discoveries were made that depicted the diffraction of light. This way light was considered as a rapid vibratory motion of a medium propagating at great speed. In these similar times Newton had opposing ideas regarding nature of light. According to him light was vibrations of corpuscles or particles with certain emission properties. Despite of this light was most successfully understood to be a phenomena of wave.
During the same 17th CE Romer performed astronomical experiments on Jupiter’s moon Io and the work of Newton and Huygens helped ascertain the speed of light to be c = 2.4 × 108 m/s and c = 2.3 × 108 m/s respectively.
In the 19th CE wave theory of light received many supporting evidences. The phenomena of interference and polarization were discovered or understood. Colors of thin films were understood and wavelength of light were determined. The wave theory successfully explained rectilinear propagation of light. In-fact it was this difficulty about wave theory which kept Newton a staunch supporter of the corpuscular theory of his rather than the wave theory. But one by one all hurdles of wave theory of light disappeared at the master strokes of many genius scientists. Similarly the need for explanation for polarization led to the correct view of light as a transverse wave rather than a longitudinal one.
Terrestrial determinations as opposed to astronomically cosmic determinations as evinced by the work of Romer became order of the day for speed of light. Fizeu by his toothed wheel method carried out an experiment that established the value of speed of light to a respectable c = 3.15 × 108 m/s. Speed of light in water was found to be reduced in comparison to speed of light value in air. This was in conflict with the corpuscular theory of light held in esteem by Newton. Not many supporters of this view remained any more in the annals of Physics, due also to the demise of the giant that Newton was, to be disproved easily or amicably.