Online lecture about treatise on light by Huygens

[PainterGuy]

Hi,

I hope that it's not wrong to ask for this kind of help in this forum.

If you don't mind, could you please help me with that name which the presenter says around 46:55 in the video below? One name he says is Cassini but the other name which he says something like "Delawear" I couldn't have been able to find anywhere. From the context of discussion, it seems both were well known astronomers.

Then, around 1:05:00 he says a name something like "Delaportaan" which I'm also unable to find any information about.

By the way, why wouldn't backward propagation of light take place because according to Huygens' theory of light each point acts as a spherical wavelet of light? That spherical wavelet is supposed to be a source of light in all directions. Is it destructive interference in the backward direction? Or, is it the assumption of theory that only forward propagation takes place? Edit: This webpage is an interesting read and tries to answer this same question: https://www.reddit.com/r/Physics/comments/35v60b/why_doesnt_huygens_principle_make_waves_that_go/ . Also, this webpage says that Huygens himself didn't have answer to this and he just assumed that only forward propagation takes place https://www.quora.com/Why-is-there-no-backward-flow-of-the-energy-during-the-propagation-of-a-wave

Thank you!

 

[Bandersnatch]

The first one is probably Philippe de La Hire - their contemporary at the Royal Academy.
The second one could be Giambattista della Porta - another Italian polymath with contributions in optics.

 

[Ibix]

Nice video of Hecht. His Optics is a classic, but I hadn't seen the man himself before.

Regarding Huygens' Principle and the "back-propagating wave" - the "back-propagating" wave is a perfect description of the incoming radiation, only time-reversed. It's an expression of the fact that all physical laws seem to be perfectly happy running in reverse. So if the only information you have is the electric field at a plane you cannot differentiate between radiation incident from one side or the other. Huygens' Principle tells you both what the incident radiation must look like to generate this field at this plane and what the outgoing radiation will look like, but not which is which. You need more information - some notion of momentum, basically - to tell the difference.

 

[vanhees71]

Well, Hecht's newer writings in AJP and Eur. J. Phys. hopefully never become "classic"... thouh is optics textbook is indeed not too bad.

 

[Ibix]

Well, Hecht's newer writings in AJP and Eur. J. Phys. hopefully never become "classic"... thouh is optics textbook is indeed not too bad.

Really? I only know of him through his textbook. Has he gone a bit odd?

 

[PainterGuy]

Thanks a lot for your help, especially @Bandersnatch!

The guy's name is Larry Hecht, at least this is what the video description says. Perhaps, he is the same Laurence Hecht being referenced in this PDF article https://larouchepub.com/eiw/public/...90827_030-how_fresnel_and_ampere_launched.pdf . On the other hand, that optics book is by Eugene Hecht.

Edit:
The video description says "2 January 1993 Larry Hecht". I'd say it was videotaped back in 1993.
Google says Eugene Hecht was "Born: December 2, 1931 (age 87 years), New York City, NY".

The guy in the video doesn't look like a 62 years old person. Eugene Hecht must have been almost 63 years old back in 1993.

 

[PainterGuy]

Hi,

Could someone please comment on that Larry Hecht and Eugene Hecht confusion from my previous post? I'm just curious. Thanks.

 

[tech99]

Nice video of Hecht. His Optics is a classic, but I hadn't seen the man himself before.

Regarding Huygens' Principle and the "back-propagating wave" - the "back-propagating" wave is a perfect description of the incoming radiation, only time-reversed. It's an expression of the fact that all physical laws seem to be perfectly happy running in reverse. So if the only information you have is the electric field at a plane you cannot differentiate between radiation incident from one side or the other. Huygens' Principle tells you both what the incident radiation must look like to generate this field at this plane and what the outgoing radiation will look like, but not which is which. You need more information - some notion of momentum, basically - to tell the difference.

But what makes a Huyghen's Source radiate away from the incoming field, one way only, and why does it have a cosine radiation pattern?

 

[Ibix]

Could someone please comment on that Larry Hecht and Eugene Hecht confusion from my previous post? I'm just curious. Thanks.

You're probably right. I didn't look at the video in a lot of detail, just noted Hecht and the obviously elderly film used and added two and two - incorrectly it would seem. To be honest, I'd forgotten textbook Hecht's first name.

But what makes a Huyghen's Source radiate away from the incoming field, one way only, and why does it have a cosine radiation pattern?

I don't think there's an explanation in terms of Huygens' Principle. It's a phenomenological model: if you apply these rules you get the answer. So as far as I'm aware the cosine dependence is based on the argument that the projected area of any elementary region of the plane decreases with the cosine of the angle. Ditto the radiation in one direction only - we observe that if radiation comes in from the right it leaves from the left so we add a rule saying this is so.

You need to go to a more sophisticated model of light (i.e. Maxwell's equations) to get something that's less jury rigged.

 

[tech99]

You're probably right. I didn't look at the video in a lot of detail, just noted Hecht and the obviously elderly film used and added two and two - incorrectly it would seem. To be honest, I'd forgotten textbook Hecht's first name.
I don't think there's an explanation in terms of Huygens' Principle. It's a phenomenological model: if you apply these rules you get the answer. So as far as I'm aware the cosine dependence is based on the argument that the projected area of any elementary region of the plane decreases with the cosine of the angle. Ditto the radiation in one direction only - we observe that if radiation comes in from the right it leaves from the left so we add a rule saying this is so.

You need to go to a more sophisticated model of light (i.e. Maxwell's equations) to get something that's less jury rigged.

I was interested to find that Thomas Young (of slits fame) had proposed in 1802 an alternative mechanism, which is that the diffracted wave originates from the edge of the obstruction. This method is known in antenna engineering today as Geometrical Theory of Diffraction. An interesting paper here: https://www.ias.ac.in/article/fulltext/pram/037/06/0457-0488