# Optics Series Lecture, Lecture – number to be assigned later.

**“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 it 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 × 10 ^{8} m/s** and

**c = 2.3 × 10**respectively.

^{8}m/sIn 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 × 10 ^{8} 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.

## Electromagnetic nature of light.

In the **19th CE** Maxwell established a relationship between electromagnetic phenomena and light wave through his experiments and theoretical work. He summarized these relationships in terms of a set of equations known as Maxwell Equations. I have a short article regarding **Maxwell Equations** and highly technical and detailed account of the same in two slide shares presentations, which you might wanna glean over if you have a high appetite for technical matters in Physics; **Electromagnetic waves** and **Electrodynamics**. Maxwell’s equations evinced the beautiful fact that light waves are nothing but the manifestations of electromagnetic field phenomena which propagate as transverse waves at the speed of light value of **c**. Hertz verified the existence of such waves by his experiments in the same century. Maxwell shown the most marvelous way to scribe the value of speed of light into the annals of Physics:

But there still were some erroneous understanding of light as a wave. It was incorrectly assumed *in the image of previous knowledge* that every wave needs a medium to propagate. Hence for light was assigned an all pervasive medium called **aether**. This was purported to be a very wispy ubiquitous medium that allows light waves to pass through it without resistance. Experiments that were performed in the **18th**, **19th** and **20th** centuries could never verify the existence of this **luminiferous aether**. Finally in **20th CE** physicists gave up the idea of this magical but untrue medium. Einstein himself was a brave scientist who supported the invalidity of such a medium in his own work.

For many practical purposes light remains as an electromagnetic wave, to this day, which propagates as a transverse wave in free space at a humongous yet finite and precise invariant speed of while in other media at reduced speeds per their properties.

But there still are many phenomena which can’t be properly explained by such an understanding as a continuous macroscopic wave. For a perfect understanding of light as is possible today, we have to resort to a theory in Physics developed in the **1920’s**. This theory is known as Quantum Mechanics.

Let us discuss the electromagnetic nature of light in more detail. The light as an electromagnetic wave is based on the understanding of electric and magnetic fields. We know that every static charge produces an electric field given as vector **E** which exerts a vector force of **F = q E **on another charge **q**. So *electrostatic charges produce as well as experience electrostatic forces*. Similarly every moving charge produces a magnetic field vector **B**. This field **B** is the reason any other charge moving at speed vector **v**, will experience a magneto-static force given by vector **B = q v × B.** Here the **× **sign represents the vector multiplication or the vector cross product between two vectors namely: **v** and **B**. Like in the case of electrostatics we summarize this by saying: *magneto-static currents produce as well as experience magneto-static forces*. A moving charge creates a magnetic field and force, also a moving charge has the qualification to experience such magnetic forces, a static charge would neither create nor experience magnetic force.

This is not all that the unification work of Maxwell entails. Every time-varying or time-dependent magnetic field can produce an electric field and thus electric current. On the contrary every time-dependent or time-varying — or **dynamic**, this is the origin of the word dynamics in electrodynamics as opposed to electrostatics, electric field can produce a magnetic field. These two phenomena are together known as electromagnetic induction. Thus we have a total of **4** phenomena, two time-independent or static and two time-dependent or dynamic effects. These **4** are summarized in the form of Maxwell’s equations which can be cast in both integral and differential forms. The first two equations are the Gauss Law, which in the case of static electric charge and electric current, express the fact that the corresponding fluxes are obtained from the bounding surface areas of the given volume enclosing the respective source charges or source currents. Note that we are concerned only about the free space where the source charge densities **ρ** and the source current densities vector **j** are null hence the right hand side is zero in **1st** equation below. In addition the right hand side of the **2nd** equation has to be zero in any case because there are no magnetic mono-poles. The **3rd** equation represents the fact that Dynamic magnetic field produces electric field — this equation is known as the Faraday’s Induction law.

Similarly the **4th** equation represents the fact that Dynamic electric fields produce magnetic fields. This equation is known as **Ampere’s circuital law**.

Thus the above **4** equations which are called as Maxwell’s equations have been cast in their integral form and applicable to free space. We can change these into their often useful differential forms by eg using advanced vector theorems: Stokes’ Theorem and Green Theorem — or by handy methods of simplifying them in one particular dimension and carrying out any required calculus. I leave such as the most honorable occupation of students: the home-work assignments. But lets us just write’em down for heaven’s sake. They retain their appellations though. Gauss law would still be called Gauss law and so on.

The central idea of an electromagnetic wave is that an accelerating charge can produce all sorts of electric and magnetic fields. Thats arbitrary. In reality it produces an electromagnetic disturbance where the **E** and **B** fields are no more independent of each other. Their direction and their magnitude are now interdependent as if someone made them follow the marriage vow: in death do we part. They are mutually perpendicular so that the wave propagation occurs in the 3rd available spatial direction, the one that is perpendicular to the plane of these mutually perpendicular **E** and **B** vectors. We will soon see how their magnitudes are co-dependent. Here is what we learn from the Maxwell Equations about light as a propagation of electromagnetic wave. **1**. The 4 Maxwell Equations produce two vector differential equations known as wave equations. We will in a future lecture learn more about wave equations. The vector **E** and vector **B** satisfy these differential equations. Thus we have two vector differential equations. Together they are known as the wave equations for light or electromagnetic wave equations. They can also be written as scalar equations such that there are **6** in total of such differential equations, one for each component of either **E** or **B** field. This has the more explicit form:

with **ψ = E _{x}, E_{y}, E_{z}, B_{x}, B_{y}, B_{z}** and

**ε**SI units and

_{0}= 8.85 × 10^{-12}**μ**SI units we have speed-of-light

_{0}= 4π × 10^{-7}**c = (**

**ε**.

_{0}μ_{0})^{-1/2}= 3 × 10^{8}m/s**2**. From Maxwell equations we can show — eg by assuming harmonic waves, (see

**this**and

**this**) that

**E**and

**B**fields are transverse in nature. The details of these on the presumption of harmonic plane waves have been presented

**here**by me, see slides

**23 – 27**, if you wanna have a look in your leisure.

**3**.

**E = cB**— eg

**E**

_{y}**= cB**.

_{z}**4**. A wave is nothing if it does not transport energy from one space-time point to another. We can get all information of transportation of energy in em waves or light, from the Maxwell’s Equations. The energy densities or energy per unit volume are denoted by a symbol

**u**so that same for electric field

**E**and magnetic field

**B**are given by the respective subscripts,

**u**and

_{E}= ε_{0}/2 E^{2}**u**. By using results of 3 above and the fact that speed of light is the inverse of square root of permeability and permittivity of free space we can easily show that:

_{B}= 1/2μ_{0}B^{2}**u**. So total energy density carried by em waves is the sum due to

_{E}= u_{B}**E**and

**B**fields. We have

**u = u**. If we define

_{E}+ u_{B }= ε_{0}E^{2 }= 1/μ_{0}B^{2}**S**to be energy per unit time carried across an unit area of cross section then considering a cylindrical geometry of energy transport by em wave we have: so we have: In general we have

**S**as a vector given by: Vector

**S**is known as

**Poynting**vector and gives the amount of energy transported by em waves in a unit time across an unit area, hence its also the power per unit area.

**5**. Irradiance. We define irradiance to be the average energy per unit area per unit time, we have already seen how it would look like, in

**lecture VII**, we also called it the radiant flux density. One can see this as the intensity as well, and evidently its the average of the Poynting vector magnitude over a given time. Poynting vector gives the instantaneous value of power per area but irradiance gives the average value, thats the only difference. For harmonic waves: We see that irradiance is proportional to the square of the electric field amplitude. We can also write the same in terms of instantaneous values of

**E**or

**B**fields instead of just the amplitude of

**E**or

**B**field. For any linear, homogeneous, isotropic dielectric medium which is result we already used in our study of interference in lecture-VII.