Speed of Light

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Nothing in the history of science can be more fascinating than the measurement of the speed of light:

As usual it was Galileo in the famous 17th century who first attempted to measure the speed of light.

He made two lamps with shades that can be opened and shut manually. He took one and gave the other to his student whom he asked to stand at the end of the street on a dark night.

You can guess what happened then. 

Galileo reported that the speed of light, as far as he could tell, is infinite.

After a century came Romer, the astronomer. 

The planet Jupiter has many tiny moons with lovely names (discovered among others by Galileo again): 

...Ganymede, Europa, Io, Callisto et al. 

(Supratim, my student in Patel Hall, showed them to me with his hand-held telescope one night in our Qrs C1-97 lawn.)

Romer found irregularities in their orbital periods and made the first approximate measurement of the speed of light (that now stands roughly at: 300,000 km/sec !!!)

That was its first astronomical measurement, followed by Bradley who made it more accurate using stellar aberration (lovely phenomenon).

Then came Fizeau who brought it down to earth. 

His technique was the same as Galileo's: Instead of shades before a lamp, he used a fast-running toothed-wheel. And in place of his student at the street-corner, he placed a mirror on a nearby hill.

It was Foucalt who brought it into a huge room using fast rotating mirrors.

And then it was the Nobel-winning Michelson who brought it right into his dark-room lab.

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Fast forward to 1995, IIT KGP, under the stewardship of Professor KL Chopra:

Lakhs of funds were given to the First Year Physics Lab to modernize it.

Tough to spend...how MANY nuts and bolts can be bought with a lakh of great rupees then!

Problem of Plenty!

http://gpsastry.blogspot.com/2014/09/problems-of-plenty-repeat-telecast.html

Word spread that lakhs were available with Prof MLM (our then HoD). 

And then there was this salesman of a reputed German Scientific Company who arrived with his gorgeous glossy manual.

Apparently, an order was placed for a modern imported setup for the measurement of speed of light on a student-optical-bench in broad daylight!!!

When the huge black-box arrived, Prof MLM asked me to open the monster with pujas, mantras, coconuts, and prayers.

I was then the exalted "Guide & Advisor" of the 4th year lab, with Prof SKR as its Lab-in-Charge and Prof BKM as the Expert.

We three carried the Betal into our lab and opened it, read the manual, and installed it on a vacant bench.

Lo and behold! 

Its principle was the same as Fizeau's. 

Lamp was replaced by a laser and the toothed wheel by an electronic chopper chopping the light beam at microwave frequencies. 

The mirror was the same as Ishani's makeup one.

All it required was a dual-beam oscilloscope that was available right in our lab.

The "experiment" was over in 15 minutes giving an accurate measurement of the fantastic speed of light...

All the student had to do was to twiddle a knob to make an ellipse on the oscilloscope screen go over into a straight line.

15 minutes!!!

But what will the student do for the next 2 hour-45 remaining minutes?

Don't ask me 😊

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