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Huygens principle incomplete?! 
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#1
May2408, 05:49 AM

P: 4

Hi all
"The Huygensprinciple recognizes that each point of an advancing wave front is in fact the center of a fresh disturbance and the source of a new train of waves; and that the advancing wave as a whole may be regarded as the sum of all the secondary waves arising from points in the medium already traversed." (source: Wikipedia) Applying this principle would mean that that the light coming from a source (eg. a laser) quickly spreads out even behind the source. One would also expect to see inteferencepatterns on a screen placed behind and not infront of a doubleslit. Obviously this is not the case (or is it??). So is there any extended huygensprinciple which describes at which angle the elementary waves emit or that gives some other explination?? Thanks in advance! 


#2
May2408, 07:28 AM

Mentor
P: 41,579

This very issue drove me nuts when I was first learning about Huygens's principle. So I feel your pain. 


#3
May2408, 07:51 AM

P: 4

Thanks Doc Al, that realy sets my mind free again.
I had been searching for days without any descent result, so I didn't expect to get such a quick and precise answer at all. 


#4
May2809, 11:31 AM

P: 5

Huygens principle incomplete?!
Hi there.
Interestingly, the introduction of the obliquity factor is an adhoc fix by Fresnel. In 1886 Kirchoff gave a full mathematical description which accounts for the behaviour that we see. The obliquity factor falls out of this theory as a limiting case. Within Kirchoff's theory, there are two types of source, and so the simplicity of Hugens' principle is lost. In addition, there is no physical analog for Kirchoff's sources... All in all a very fascinating problem. I'd recommend reading this paper: http://wwwee.stanford.edu/~dabm/146.pdf Or D. Miller, "Huygens’s wave propagation principle corrected," Opt. Lett. 16, 13701372 (1991). if the link doesn't work. Good luck! 


#5
Mar1711, 07:16 PM

P: 3

I always wondered how this affects the explanation of reflection.
If you consider the reflection line representing the mirror, actually none of the physical properties of a mirror enters the proof at any point. The proof therefore applies to any imagined line. So why aren't there reflected waves all over the place? 


#6
Mar1711, 08:59 PM

P: 969

How does this change for real waves? What I mean by real waves is something like water waves, where there is actually something that is vibrating, so that the sources on the wavefront are real: do these sources generate secondary waves that are isotropic?
Take 2slit diffraction of water waves. The screen blocks the waves, but the slits can be seen as sources for a new wave because there is actually water at the slits that vibrate (like dipping your finger in a pond at the slits). Do the water at the slits generate backwards waves? 


#7
Mar1711, 09:09 PM

P: 3




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