# Why in Huygens–Fresnel, Point source should be as small as wavelength?

1. Mar 21, 2014

### payam

In Huygens–Fresnel principle, to configure the diffraction pattern after an aperture, we need to consider each point on aperture as a point source of light.
I need to make a simulation for diffraction. Here I will need to introduce the size of each point source.
my question is why the size of these point sources should be as large as the used wavelength ?

2. Mar 21, 2014

### dauto

Because if they were any larger they would be extended sources - not point sources - affecting the results, and if they were smaller your simulation would take longer than necessary to run. I would use a size a bit smaller to be on the safe side. A quarter wavelength seems appropriate.

3. Mar 21, 2014

### payam

Thanks Dauto.
So there is a definition for point source in terms of angular divergence?
what is the definition of a point source?

4. Mar 21, 2014

### dauto

A point source is a source significantly smaller than the wavelength of the radiation being emitted. Otherwise it is an extended source.

5. Mar 21, 2014

### Staff: Mentor

By definition, a point has zero size, doesn't it?

6. Mar 21, 2014

### dauto

Yes, it does, but (s)he is talking about some kind of simulation. It's not exactly clear how the simulation works but it is conceivable that an infinite number of point sources could be replaced with a finite number of extended sources and as long as the sizes are comparable to the wavelength or smaller the simulation might produce viable results. Again, not knowing exactly how the simulation is supposed to work, I'm going out on a limb trying to make sense of it. May be the OP should explain the simulation better?