Wrong question in GATE 2018 physics?

I think the above question asked in GATE 2018 (physics) is wrong.

Any vector has two components. The component perpendicular to the parity axis has even parity and the parallel component to the axis has odd parity.

The opposite is true for axial vectors.

E, A vectors.
B, L axial vectors.

The correct answer per gate exam body is E, A. Why not B and L? It’s an arbitrary situation and perpendicular components of these fields will have odd parity.

Electromagnetic Nature of Light — A brief history of light

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.

Interference of two plane harmonic waves.

Optics Series Lecture, Lecture – VII.

“Conditions of interference, Interference of two plane harmonic waves.”

This lecture was delivered on 7th February in a lecture session of 1 and 1/2 hours. This lecture was delivered to Physics elective students but intended as a lecture towards Honors students at a later date.

Electromagnetic Waves.
Light is an electromagnetic wave. In-fact its a transverse electromagnetic wave which means the oscillation of E and B fields produces light which propagates in a direction that is perpendicular to the plane that contains the E and B fields. In other words E, B and k the vector that denotes the direction of light propagation, are mutually perpendicular vectors.

We will study these details in a later intended lecture. EM waves are not only transverse waves but also vector waves, that is; E and B are vector fields whose undulation is summarized as light.

Light is a general name for all EM waves but visible light is that particular part of EM waves which has frequency of wave such that the wavelength varies from approximately 400 – 700 nm. In vacuum — only in vacuum, light always moves at a fixed speed: namely c = 3×108 m/s. Therefore light whose wavelength lies between 400 – 700 nm is called as visible light: we can write in vacuum c = νλ.

Light as a transverse wave phenomenon of vector fields is comprehensively described by four equations known as Maxwell’s Equations. The Maxwell’s Equations are a summary of important and fundamental laws of electricity and magnetism — together called as electromagnetism, such as Gauss Law and Ampere’s Law. These equations produce the wave equation of motion, a linear, homogeneous, 2nd order differential equation that we will study a few lectures afterwards.

If you are quite serious and technically well equipped though, you can have a glimpse of it all — and may be work out to your satisfaction, by following the link to my slide-share presentations. There are many other important Physics concepts that are worked out in great detail, in those slide-share presentations by me. eg check: Electromagnetic Waves.

Let us therefore write the wave equation of motion, where the 3 components of E field — such as Ex, Ey or Ez or the 3 components of B field such as Bx, By, Bz, are denoted as ψ chosen anyone at one time. eg we can chose Ex = ψ. In general we have: 

The Maxwell’s Equations, from nature to instruments.

The beauty of Maxwell’s equations can be seen in how it helps us understand nature as well as instruments, at the same time. Medical devices are simply an advanced understanding that began with understanding electromagnetic waves through Maxwell’s equations.

Each of the following 4 equations has a different name, by which we call’em, but together they are called as the Maxwell’s equations. Together they constitute what I am inspired to say; the golden equations of Physics. If we do some easy tricks they will be converted into whats called as the Wave Equations (of motion) ! Yes, they describe the wave behavior “fully”.

— By that I don’t mean sound waves, but any sort of waves that move at the speed of light. Sound waves are ordinary pressure oscillations, that travel much slower than even rockets.
The 4 equations therefore describe how electromagnetic waves are created and broadcast. Hence TV radio and satellite communication were understood because these 4 equations were understood.

First two are time-independent or static equations.

The first equation is known as Gauss’s law of electrostatics and says “Electric fields (E), are a result of sources of electrostatic charge”.

The 2nd equation is analogous and called as Gauss’s law of static magnetic field. But it says “apparently there are no sources of magneto-static charge or single magnetic pole from which the magnetic field B is created”.

Then how are magnetic fields created? We needed to know further to find the answer. Lets look at the 3rd and 4th equations.