Optics from Feynman

Feynman Physics Lectures V1 [Ch 26]

Here’s a chapter from Feynman’s Lectures on Physics. It’s an illuminating chapter on optics, which explains what you will study in your course under Geometrical Optics. A must read!


Response to your Questions

I will begin with Archana’s question about the detection of ultrasonic waves.

How are ultra sonic waves detected?

There are many ways to detect ultrasonic waves and the choice of the method will depend on the specific situation and the need in an application. One of the most commonly used method is based on piezoelectric effect. Now there are some crystals which develop a small electric potential difference if you apply some presure across their faces and the potential difference depends on the pressure. To detect an ultrasonic waves (and study its properties), a piezoelectric crystal is placed in that region. The ultrasonic waves, being mechanical waves in nature, cause pressure variations across this crystal and the crystal produces a changing potential difference which is picked up by an electronic circuit containing an amplifier which amplifies the electrical signal. Well that’s the story in brief. By the way this is not part of the ICSE course.

Since Parth’s questions are about sound, let me take those up now.

How are sound waves transverse waves in a solid medium?

Solids can apply what is called a shear stress while fluids (liquids and gases) cannot. It is because of the shear stress that the solids can transmit mechanical waves in transverse waves as well. Even if you do not understand all this, I think if you can look at the small animation (right in the beginning) on this page, that would answer your question: https://www.nde-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/modepropagation.htm

If the pressure increase, wouldn’t the no. of molecules per a standard volume increase? If, true then shouldn’t that increase the speed of sound since that is the case in solids. Sound is faster in solids as they are denser right?

Yes, the pressure will increase the density of a gas but an increase in density leads to a decrease in the speed of sound.Speed of sound in gases depends on two factors: (a) Density {speed is inversely proportional to the square root of density}, (b) Elasticity {speed is directly proportional to the square root of elasticity}. An increase in pressure increases increases both these quantities and hence no change in the speed of sound.

Solids transmit sound faster because they have a much higher elasticity.

The textbook says that transverse waves travel only in a media with rigidity then how is it that light travel in vacuum?

I don’t have the textbopok with me but if that’s what it says (that transverse waves travel only in a media with rigidity), that’s incorrect. I think the textbook might be referring only to mechanical waves.

What factors contribute to the amount of sound reflected?

Surface area and the nature of the reflecting surface.

In sonar if the receiver and transmitter are placed close together then wouldn’t it be a disadvantage as those waves follow the law of reflection and so if the object is at an angle then it wouldn’t be detected.

Yes, but the objects these SONARs are really looking for are large in size and definitely rough in texture and hence the reflected waves will go in all directions the way light gets reflected off a wall in room in all directions because of its surface.

Now to Srishti’s questions:

In transmission of power from the generating station to consumer why do they step up from 11 kV to 132 kV and then step down to 33 kV , they can directly step up to 33 kV.

You had mentioned kW instead of kV in your question. I have made that correction. Now to answer your question, the voltage is increased by step-up transformers to 132 kV so that the current is reduced by the same ration (132:11, i.e. 12:1). Then the power is transmitted at this high voltage from the power station to different towns over long distances. The advantage of transmitting power at such a low current is that you reduce the I^2 R power loss in the conducting wires (which are kilometres long and hence have large resistance). Once the wires have reached the town, the voltage is stepped down in stages for different users.

PK why should the fuse always be connected before the appliance when current stops when it’s placed anywhere in the circuit?

Ig the fuse wire is after the appliance (i.e. in neutral wire), the current will stop hen the fuse melts but the device (or its switch) will remain connected to the live wire. That puts the user at risk of getting electrical shock.

From Feynman: Fun to Imagine

I don’t remember talking to you all much about Richard Feynman. He was an amazing physicist from the last century and is considered by someone as the second brightest minds of the past century. The brightest mind ‘awarded’ by the same ‘someone’ goes to (no! it’s not Einstein!) a mathematician John von Neumann.

Coming back to Feynman: besides doing a lot of work in theoretical physics, Feynman is also well known as a popularizer of science. See the following excerpt from Wikipedia:

Feynman was a keen popularizer of physics through both books and lectures, notably a 1959 talk on top-down nanotechnology called There’s Plenty of Room at the Bottom and The Feynman Lectures on Physics. Feynman also became known through his semi-autobiographical books (Surely You’re Joking, Mr. Feynman! and What Do You Care What Other People Think?) and books written about him, such as Tuva or Bust! [These books are available in the library and I would highly recommend Surely You’re Joking… and What Do You Care… even to the people who are not physics-enthusiasts!]

Recently I found this series of short videos (around 10 minutes each) in which Feynman talks about various physical phenomena. The series is aptly named as “Fun to Imagine”. Here’s the link for the whole series on Youtube: http://www.youtube.com/watch?v=y7h4OtFDnYE&playnext=1&list=PL04B3F5636096478C

I recommend watching the following video (related to electricity and magnetism):

For students of class 10B

Hope you would have revised Electromagnetism by now. I would like you all to take this online multiple-choice quiz on electromagnetism: http://tinyurl.com/elecmagquiz

Hope it would help you revise some concepts. Though I have designed the quiz in such a way that you would get a feedback on your performance in your mail box, if you are still left with some doubt/question, leave it as a question in the Comments for this post. I will respond to that.

Prep work for Modern Physics

I would like you all to prepare a little bit for your course on Modern Physics. I feel there is some part of the history of physics that is usually not covered in class 8-9 physics or chemistry classes and it is essential to appreciate the bit that you have to learn under Modern Physics in class 10. So what I would like you all to do is to watch this video of a lecture (see below) from a course on Principle of Chemical Science from MIT. Watch the video from 3:00 minutes to 32:00 minutes. As the title of the lecture suggests, the first part covers the experiments that were behind the discovery of electron and nucleus of the atom.

In case, you cannot see the video here, click on this link to watch the video in another window.



It’s been more than a week into the vacation. I suspect there must be an expectation at home now that you spend some time with your academic work. So unless your family is busy looking forward to Diwali, I guess it’s the right time for me to pop up and ask you to do some physics.

I am sorry I couldn’t finish the correction of papers in time. You all generally have done well. We will discuss the paper in detail once you are back here. I also want to clarify here that the mark sheet which you will receive (in a few days) will have your theory (80) plus your lab work (20) marks combined for physics.

I would like you all to begin by revising Optics and Electromagnetism (for 10 B students). Read your textbook thoroughly.

For the coming term, I would like you all to read through the chapter on Sound and Modern Physics. Class 10 B students should also read through the first two chapters of Mechanics from the textbook.

I will also post some worksheets or may be test papers for revision work.