Huygens Principle: Light & Diffraction Explained

In summary, the conversation discusses the concept of diffraction in relation to wavefront and aperture size. It is agreed that diffraction occurs even with a wide aperture, however, it is only significant near the edges. The reason for this is explained as the superposition of point sources, with reasonable intensity only in the forward direction. The conversation also touches on the application of Huygens Principle in these scenarios.
  • #1
dsanz
42
2
Hello, I have the following question:

Physics books say that in order to see diffraction you need a wave to encounter an object that is comparable in size to the wavelength. This is presumably the reason why light does not bend through an opened door but sound does. My question is, what happens to Huygens Principle in the light scenario? If we can always consider the wavefront as another light source, wouldn't this mean that when light encounters ANY slit, no matter its size, the light will spread out in all directions? Does Huygens Principle simply not apply in some cases??

Thank you.
 
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  • #2
You do get diffraction with a wide aperture. The effects are significant only near the edges. In fact, you also get diffraction with only a single edge, i.e. a semi-infinitely wide aperture. A Google search for "knife edge diffraction" might turn up some pictures.

The following page has a Java applet that simulates diffraction for slits, circular and rectangular apertures, and knife edges (semi-infinite planes). You can vary the width of the aperture and observe the results.

http://www.falstad.com/diffraction/
 
  • #3
Thanks for your reply jtbell. I am still confused as to why then does the aperture have to be comparable in size to the wavelength of the wave. If diffraction always occurs anyway, then why is it that light is supposedly not diffracted through a door opening?
 
  • #4
I think that is just a rule of thumb for when the effect becomes apparent.
 
  • #5
dsanz said:
If we can always consider the wavefront as another light source, wouldn't this mean that when light encounters ANY slit, no matter its size, the light will spread out in all directions? Does Huygens Principle simply not apply in some cases??

You can consider EACH POINT of the wavefront as another light source. For a narrow slit, this leaves you with a point source. For a door, you get a huge number of point sources. The superposition of these will only give reasonable intensity in the forward direction. In the other directions the contributions of all the point sources will just cancel out when superposed as the relative phase is pretty random in these directions if the slit is larger than a wavelength.
 
  • #6
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  • #7
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1. What is Huygens Principle?

Huygens Principle is a theory that explains how light propagates through space. It states that every point on a wavefront can be considered as a source of secondary spherical wavelets, and the sum of these wavelets determines the shape of the new wavefront.

2. How does Huygens Principle explain the behavior of light?

According to Huygens Principle, light behaves as a wave and exhibits diffraction, interference, and reflection. This means that light can bend, spread out, and interact with other light waves as it propagates through space.

3. What is diffraction and how does Huygens Principle relate to it?

Diffraction is the bending of light around obstacles or through small openings. Huygens Principle explains diffraction by stating that each point on a wavefront can be considered as a source of secondary wavelets, which can interfere with each other and create a diffraction pattern.

4. How is Huygens Principle used in practical applications?

Huygens Principle is used in various practical applications, such as in the design of optical instruments like microscopes and telescopes. It is also used in the field of optics for studying the behavior of light in different mediums and for developing new technologies such as holography.

5. What are the limitations of Huygens Principle?

Huygens Principle is a simplified model and does not fully explain all the behaviors of light, such as the photoelectric effect and the wave-particle duality. It also does not take into account the effects of quantum mechanics. Therefore, it is not a comprehensive explanation of light and has its limitations in certain scenarios.

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