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Physical Optics

Physical Optics

by MRIDUL KANTI ROY CHOWDHURY -
Number of replies: 4

According to Huygen's principle, every point becomes a source of secondary wavelets sending such wavelets in all directions when reached by a progressive wave travelling in that elastic medium and at any instant, the shape of the new wave can be found out by drawing a surface enveloping all the secondary wavelets.

I have seen in all the books, that the secondary wavelets travel only in forward direction through, according to Huygen's Principle, they should travel backwards too. Can anyone please point out the reason behind it?

In reply to MRIDUL KANTI ROY CHOWDHURY

Re: Physical Optics

by Ankul Garg -
Mridul I can tell you the whole picture of how wavefronts are formed. I think you will get the answer to your prob. in that only.

IT WOULD BE BETTER IF YOU SIMULTANEOUSLY DRAW THE DIAGRAMS ALSO.
AND TRY MAKING THE WHOLE PICTURE IN ONE DIAGRAM ONLY.

Consider an infinite no. of collinear point sources of sound waves. Now every point source will form spherical wavefront. Let r be the radius of all the infinite spherical wavefronts at any time t. These are primary wavefronts. Now one can imagine a hollow cylinder having axis as the line of point sources, with radius r which is touching all these primary wavefronts. This hollow cylinder is the secondary wavelet.
Now, consider the curved edges of the secondary wavelet. All the points on one such edge can be considered as point sources of sound waves. Again they will form primary wavefronts(spherical). Draw such spherical wavefronts on both sides of the cylinder. Now again imagine a hollow cylinder with axis as the original line containing the point sources, and touching all the newly formed primary wavefronts. This is again a secondary wavelet.
This goes on. And i think this can go on in forward direction only.
Did you get what i have tried to explain you?? Reply please
In reply to Ankul Garg

Re: Physical Optics

by MRIDUL KANTI ROY CHOWDHURY -

Ankul, thanks for your explanation but in the picture you wanted me to draw, I found out that the cylinders you are talking about extend in all direction, so they will also extend towards the original linear source of disturbance and again, in the back part of the cylinder i.e. the part facing the original linear source, the points on it will become sources of new disturbance. So , new cylinders will be formed which will also have their 'back' part facing the original source. So , the wave practically both converges and diverges at the same time. This agrees to the energy distribution which we see in everyday life where the intensity of light decreases gradually as the wave proceeds further from the source.

So, I really can't understand how they can only travel forward. May be I'm unable in uptake, but I'd be grateful if you can explain in a plainer way.

Please reply with a plainer explanation and try out point sources too.

Please reply soon.

In reply to MRIDUL KANTI ROY CHOWDHURY

Re: Physical Optics

by Manish Verma -
Great question!

Huygen did not consider such wavefronts as they would lead to light travelling in backward direction. Science matures over time and many theories in the past needed refinements (recall Bohr's model for example). This was the weakness of Huygen's model. Despite weaknesses Huygen could prove the law of reflection & refraction. To quote Schwartz, ".....Thus we will begin by throwing out Huygens' principle completely; later we will see that it actually does give the right answer for the wrong reasons". Though I won't say that you can throw it out completely, especially when it is in your syllabus smile.
In reply to Manish Verma

Re: Physical Optics

by MRIDUL KANTI ROY CHOWDHURY -

Thank you for your opinion sir. I have just one more question. I have heard that there exists an equation which comes out of experimental diffractions which is as follows -

amplitude of the wave = (1+cos theta[could not find symbol of it])/2.

Where theta is the angle between the line of wave propagation and the direction we consider.

According to this, in backwards direction, theta = pie. So, amplitude = 0.

Could you please explain the deduction of this formula to me?