Speed of Light

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The other day there was this news item (later retracted?) that they found neutrinos traveling faster than light.

That recalled my fondness for the topic of Measurement of Speed of Light c. I used to revel in teaching this to first years during my youth at KGP.

If I recall right, that great soul called Galileo made two lamps with shades, gave one to his Assistant and kept one to himself. And one night asked his Assistant to go to the other end of the street and open his shade as soon as he sees light from his own lamp. And counted the seconds, possibly on his pulse. And decided that the speed of light is infinite or nearly so.

This is what I call Cheek...an essential qualification for a Physicist.

And then came Romer who did measure it by watching the moons of Jupiter. And then Bradley using stellar aberration, a fun topic in itself. And then coming down to Earth from the Heavens, Fizeau and his toothed wheel and two hillocks. And then Foucalt bringing it within the four walls of his lab by his rotating mirror. And then Michelson's celebrated interferometric measurements.

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Once upon a time UG students started grumbling that their Phy Lab was most boring and they were doing mesozoic experiments measuring surface tension and viscosity and were being ridiculed wherever they went.

And one day DB ran to me and told me that there is a new experiment in the third year lab on the 'measurement of the speed of light'.

I was rather credulous and also incredulous. And surreptitiously visited the third year lab when no one was there but Basanto-da and asked him to show me the thing.

It turned out to be truly revolutionary.

There was this coil whose parameters like the number of turns, radius etc were known. And students were using an AC Bridge to measure its self-inductance L and, using the formula for L, getting the mu-not of vacuum.

Now, now, using the Halliday-Resnick value of epsilon-not, it is a matter of simple calculation to get the speed of light c in, you know, vacuum.

However, it is not that simple to please IIT students by such gimmicks.

And then the new Physicist-Director sanctioned a fantastic sum of twenty lakh rupees, give or take five lakhs, for the Modernization of the UG Phy Labs.

And show him.

There was this scramble and a big German Company sent its salesman with their latest literature. MLM was the HoD then and apparently placed an order for an experimental set-up to measure the speed of light in the second year laboratory. I was then the Big Boss of the fourth year lab. I got to know about this when MLM came to my room and said that the Pandora's Box has arrived from Germany and could I please open it, install it, test it, measure the speed of the damn thing, and show him how (one lakh rupees have to be accounted for urgently).

I asked BKM the Inside Story. He told me that the thing they sent was not the thing they promised to send...some accessories in the list were missing.

I told MLM that I wouldn't like to go to the second year lab (the lab-in-charge may get angry); and he should send the Box for its deflowering to the fourth year lab where BKM (the present Dean) and SKR (the present HoD) were available all the while. He said ok.

We three opened the Box and after some fiddling discovered that the Germans sent their latest model that no longer needed some of the listed accessories of the older model quoted.

It took about half an hour. And my delight knew no bounds. It was indeed a time-of-flight measurement using a laser beam. There was this wonderful optical bench at one end of which is shot a He-Ne laser beam that is electronically modulated much like Fizeau's toothed wheel cutter. And a mirror at the other end. The reflected beam is mixed with the outgoing beam, demodulated, and the phase difference (due to the time of travel) between the two is measured by watching the Lissajous Ellipse on the screen of an oscilloscope. Just move the mirror on the optical bench till you go over from a straight line through ellipse, circle, ellipse and straight line back.

We felt like Fizeau for once!

The (chopped) light beam can be seen with the naked eye going back and forth, in the mind's eye. And it was sensitive enough for UGs.

Heil Ho Deutschland!


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