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## Doppler Effect

Back in 11th grade i used to cycle 10kms to listen to some lectures given by an old Physics professor, Mr M.L.Ogalapurkar, he has easily been one of the best professors who has taught me physics. So i would absolutely love the two hours spent there every Friday and Saturday.

One fine day he taught us Doppler effect and took some problems, then extended the concept to include the asymmetric effect i.e in light.
Then he took a simple question. This is question number 5.233 in I.E Irodov which says –
(i have the 1988 edition of “problems in general physics“)
“How fast should a car move for the driver to perceive a red traffic light as a green one?”
on reading this question his face lighted up and he put down the book. Slowly took the chair (he limps) and started.

There was this physics professor, he always got late. So as usual this fine morning he was late again, so he just took one Physics book – his favourite and threw it in his car and sped off to the university.
Now late, he knew he was over-speeding and hoped no cops stop him. Just as he hoped he broke a traffic light in his hurry. He could see it was red but ignored it as anyway the roads were empty. Just as he was starting to get a relaxed look on his face on breaking the signal, a cop signaled him to stop and then he had to.
The cop said “Sir, you have broken the signal, i must ticket you”.
the professor thought for a minute and his hand moving towards the wallet stopped and then he thought
“this is a police cop what will he know about physics?” and he said
“Officer, i have not broken any signal, i saw the traffic signal as Green not as Red”
The Cop: “Well are you trying to kid me? how is that possible”
Professor: “Blame physics for it, see equation 17.68 here in this book makes this possible and i just experienced it! If you travel fast enough then the frequency appears altered and Red appears Green”
now confused the cop said “Well, i don’t know but i can’t let you go just like that, you can say all this in the court”

in the courtroom the judge asked him the same thing.
“Professor i got a complaint that you broke the signal?”
the professor gave the same answer as he gave to the cop.
the Judge too got confused, and said
“i have no qualification in Physics so i can’t really say anything on this. i shall consult some qualified person and then pass the judgment”
and just as he was to adjourn the proceedings a person seated there got up and said
“Judge, i am a qualified person and i am working in the JPL, will you consider my opinion”
the judge saw his i-card and said
“your opinion may be considered as valid as you are qualified to have an opinion on this”
the JPL scientist then said “Sir, the professor is absolutely right! this effect is called the Doppler effect and it happens”
and then he gave him the example of a whistling train.
The judge still not convinced says “Well but how does that make a Red signal appear Green?”
The scientist got up and took the Physics book the professor was carrying, using equation 17.68 he worked out the problem and then showed the judge his calculations!
he said “judge, see this is possible i have worked out that a Red signal will appear green when the car is moving at almost thirty percent of ‘c’. and so the professor is absolutely right!”
the judge studies the problem and then speaks out

“well. professor you are absolved of the charge that you broke a signal”
The professor now very relieved gets ready to leave, just as he turns..
the judge speaks out

“but you have exceeded the speed limit!”

*SIGH*

PS: I do not know the exact source for this joke, i just remember Dr M.L.Ogalapurkars words and obviously not exactly as how he told me.

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### One Response

1. on May 18, 2011 at 6:09 am | Reply Shishir Gupta

Hi Shubhendu,

I too remember this story told by sir. How much I remember in the end, once the professor is charged for over-speeding, the scientist comes up to him and says, “sir I was a student of yours’ a few years back. And you had given me a mark less that one of your relatives as you wanted him to top the class. And the explanation for that mark being cut was that I had not understood the Doppler Effect properly. Now you would agree with me that I have finally understood it.”

I think it was something like this towards the end, but not very sure of this. Also I don’t remember how this started. If I am not wrong, it was an example given by sir to explain the concept.

Anyways, it was great to read this story up here, and I did not know it was here from such a long time.

-Shishir