# Huygens Principle

1. Apr 1, 2013

### dsanz

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.

2. Apr 1, 2013

### Staff: Mentor

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.

3. Apr 2, 2013

### dsanz

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. Apr 2, 2013

### ModusPwnd

I think that is just a rule of thumb for when the effect becomes apparent.

5. Apr 2, 2013

### Cthugha

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. Apr 2, 2013

### dsanz

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7. Apr 2, 2013

### Cthugha

Please do not tell my girlfriend.

8. Apr 2, 2013

### dsanz

I am a straight guy, but sure, no problem