Beauty Spot

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This morning I was in for a Shock. Or rather a Betrayal.

For the past ten days, Aniket has been darkly hinting that he, his Mrs, and their cute 16-month-old-Beany were planning to attack our home to look up our 5-month-old-Ishani.

I ignored it. Hint turned an e-mail threat. As a rule, if any of my 'clan' writes a 3-sentence mail, I compose and send a 30-line reply at once. Exuberance. But I dithered for 3 days; and wrote a 3-word-reply: "Let Heat Abate".

I then got an ultimatum that this weekend would be my "Ides of May". I suggested: "Why do you take the trouble? We would visit you instead". He then exclaimed: "How would THAT make for a badly-needed outing for my wife and kid?"

This type of dialogue always reminds me of the famous repartee by the Village Farmer to the City Psychiatrist.

Anyway, everyone knows that Aniket and me are born slippery-customers. Ask KK. When we try to do our Slippery Act with the Rest of the World, we are in it together, and aid and encourage each other by word and gesture.

But when we confront each other it was a close thing. It is like that Army & Navy Store, which sells that Spear which can pierce any Shield, side by side with that Shield which no Spear can pierce.

Or to change the figure, we are like two Sumo wrestlers jumping into the Ring just out out of soak-pits of castor oil (me) and honey (Aniket). Like that kid in the Slumdog who I am told surfaced out of a soak-pit of Chocolate Gravy.

This round went to Aniket.

After seating himself in our AC Bedroom, he broke the news that he read my latest blog posted this morning. The whole idea of posting blogs in the early hours and then going to bed is that after waking up at noon and checking it by the daylight I can edit, shorten or delete it safely before the readers, if any, can access it as usual in the evening.

But Aniket had to read it first thing in the morning and confront me in person.

My whole idea of blogging after retirement is this: I am a born story-teller. It runs in the family. Ask my nephews and nieces. Even now they flock to me, ripe souls they by now are. The essence of good story-telling is a judicious and audacious mix of euphemism and hyperbole deep-fried in pure gul oil.

It was my misfortune that instead of teaching Economics, I had to teach Physics all my life for a living. Now Physics has to do with 'facts' (mostly supported by incorrigible experiments). In my Final Semester at KGP, I wrote an Application to the Standing(?) Time Table Committee headed by Somanth/Sayan (as the case maybe) that I be allowed to teach one final semester of pure gul. They routinely sent me a Standard Proforma asking for Qualifications & Experience in the relevant field, attaching cited publications (not less than 3 in the past six months). Which I did. After the Scrutiny Jury met in a closed-door session for over an hour, they declared (orally, not to offend me or commit themselves): "Although even a convict in the death-row is permitted his last wish, we regret to convey to you that we found unfortunately that you are not qualified to teach String Theory".

Then and there I decided to take revenge on the entire Physics Community by narrating absolutely mythical stories in fertile fields of Physics by joining the celebrated Blogosphere (Hail Blogosphere!).

And this morning Aniket announces on empty stomach that he found my latest blog full of references to his Idol's pretty Idol, and waded through the relevant quoted Volumes of Sommerfeld and checked page by page, para by para, and foot-note by foot-note to check the veracity of my tall claims.

This, as Mark Antony would cry: "is the most unkindest cut of all". For, it was this same kid who encouraged me to 'forget facts' while blogging and wrote a famous Foreword to that effect.

A good story has a skeleton of Truth surrounded by a flesh of Fiction. Both are necessary.

Take, for instance, Madhuri Dixit. I mean, figuratively. Everyone knows that MF Hussain in Dubai and gps in Hyderabad are the sole surviving undying Senior Citizen Fans of Madhuri.

Before I forget: A few years ago, my son tried to wake me up of a lazy afternoon nap, shaking me and announcing that he has just then got hold of a pirated CD of Devdas and wanted me to watch it with him. He recalls that the last film we watched together several times in a day was 'The Great Train Robbery'. He was asking for a repeat.

I asked him who plays Devda. He replied: "Shah Rukh Khan". "And who plays the chap who 'drinks' him". "Jackie Shroff". I say: "Devdas is a deep and dark character. Shah Rukh is a joker. They should switch sides". And I switch sides and try to sleep off. He then shakes me and cries: "Madhuri is there". I sort of get up. "And who else". "Ash". "Who plays Paro and who that nauch-girl?" "Ash & Madhuri resply". I shout: "It should be the other way round!". And go back to bed.

Now, her skeleton alone would need lots of imagination to appreciate Madhuri (by laymen I mean). On the other hand her flesh alone would be like that of a hilsa without its delicious bones (sorry!). Both are required in proper arrangement for a story to gel.

Anyway, Aniket didn't say in so many words that my Sommerfeld's quotes were 'unfounded'.

Instead, he suggested, that by way of mitigation, I should 'retell' that story of Poisson Spot in my next blogpost.

'Retell' meant that I had told him this story once or more than once, with enriching embellishments each time around.

Having lived in Bengal for 4 juicy decades, I grasped two inalienable features of Bengali youth (upto 80 years of age):

(1) They are each fans of some or other hot shot fairly big; and they fight as to 'who is bigger' (now don't ask me Aniket, to 'retell' the 'who is bigger' triad) tooth and nail.

(2) They HAVE got to 'rank' everyone in every field, and each field by the other field. (SSG approves my take and even cited some song by some turn-coat Bengali poet as proof).

SDM, as I told you yesterday was an undying fan of Sommerfeld.

DB was a deathless fan of Maxwell.

And me a perpetual fan of Fresnel, from my tender age of 18 till now. Every time I look at his work, I 'discover' something new, like that White Knight (?) in Alice.

I don't know if you have seen the picture of Fresnel in, say, Wikipedia. It has that romantic agony writ on it just like Keats (sorry, Keats didn't have to face 'experimental evidence'; he could shoot crap).

Fresnel was a slow learner. Couldn't speak coherently till 8 or so. And died, like Keats, of consumption, at the tender age of 38 or so.

In between, he accomplished so much that Maxwell only had to put in the Math and the daring Vacuum Displacement Current.

And, being a fan, I can write a small tome on him now if only I can quit blogging.

Anyway, like Keats, he died in penury and almost anonymity. All revolutionaries have to face this (us precluded).

At the tender age of 18, he submitted to the Academie of France (or some such Body) his theory of half-period zones, now known as Fresnel Diffraction, for approval.

Now, Theoretical Physicists are like batsmen and experimentalists are like bowlers. A bowler may be hit for for successive fours and sixes over after over; nothing much happens to him.

On the other hand, the batsman has only one chance. One miscued hook, and he is OUT!

Likewise, someone's Theory may fit a hundred experimental facts; but if it can't explain the next one, it is declared out!

Poisson, of the Poisson Equation, was asked to scrutinize Fresnels' Paper. All a referee has to do to shoot down a Theory is to point out or invent ONE absurd experimental consequence which that Theory contradicts (we all had tried that, in a small way, in our Refereedom).

Overnight Poisson said that Fresnel's beautiful theory is bullshit, because it predicts that a beam of light falling on an opaque disk should have a bright spot at the center of its dark shadow. Period.

Fresnel looked into Poisson's objection and got gloomy because that is exactly the absurd rsult that his Half-Peiord Zone Theory predicts.

And went into deep depression with accompanying suicidal tendencies. At such a tender age!

Everyone had a good laugh (any upstart is anathema for the Establishment).

Except Fresnel's friend, Arago.

He said to himself: "Let me see".

And he SAW the bright spot at the center of the dark shadow exactly as Fresnel's theory predicted and pointed out by Poisson.

Arago's Paper with his live demos jerked up the Establishment.

And it decided that the newly discovered Beauty Spot be called, not 'Fresnel Spot' nor 'Arago Spot', but 'Poisson Spot', because he was who first suggested its 'existence'.

Take heart, young ones, keep poking holes in established theories...you may have your very own 'Beauty Spot'.

Coming to serious Physics (Ahem!):

The opaque disk has to be of the size of the wavelength of light. That is too tiny for light. Hence Arago ought to get his due credit.

Now, we have microwaves and Big Zone Plates for Microwaves, easily made in any lab.

And, since the advent of thin grating grooves, like the video cassettes, we can have Fresnel Lenses of decimeter-size containing millions of half-period zones alternately opaque and transparent that give such a bright spot acting like Zone Plates for light that burn holes in metal plates just by playing with them as we used to do with big convex lenses in sunlight in our childhood. Only please be careful not to gift one to your kids.

Again, when I was teaching Zone Plate during my last semester at KGP, I discovered that Zone Plates ought to produce 'virtual' images in addition to real images found in text books.

Any takers?

Just before blogging this artcle an idea dawned on me:

Everyone knows that any system that gives a bright point spot for a bright point source can act as a 'camera': Like the pin-hole camera.

This indeed is found in Jenkins and White, which has a photo of the 'image' of a coin that has Woodrow Wilson on it, produced by its collection of Poisson's spots on a screen, via a half-period sized opaque disk.

Now, we know that a pin-hole camera does the same, as beautifully.

The advantage of these two 'imaging systems' is that they are absolutely free from spherical aberration. That's fine.

What occurred to me was that these two systems: viz: (1) the pin-hole camera, and (2) the Arago opaque disk form COMPLEMENTARY SYSTEMS: Meaning, interchange the transparent and opaque portions of the 'pin-hole camera'; And you get the 'Arago disk-camera'.

Remember Babinet's Principle: "Complementary systems produce the same Diffraction Patterns" under suitable conditions.

My take is: Had Fresnel known Babinet's Principle, he could have knocked down Poisson with this feather: "Of course, there will be a bright spot, just like in a pin-hole camera!"

Am I right?

Some Physics at last!


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