Diffraction and aperture size

[Ahmed Abdullah]

We know that to observe diffraction the width of the apertures in the grating must be of the order of the wave-length used. Would we observe diffraction if the apertures are significantly narrower than the order of wavelength used? Or what would be the difference?

I think we would not observe diffraction at all. Because at the mentioned case, the aperture can be considered as a point source. But I have confusions and don't have clear-cut idea about why it should be like that.

[turin]

Don't confuse two sources of diffraction pattern. For a grating, it is actually better to have small apertures. The diffraction pattern is actually an interference pattern, and it results from the superposition of the radiation from multiple aperatures that are usually treated as isotropic radiators. A true diffraction pattern (due to diffraction) will also occur from a single aperture whose size is on the order of a wavelength. A pattern will result from a grating with apertures on the order of a wavelength, but calculating this pattern is a bit more complicated (you have to do a convolution). There should be negligible diffraction pattern from a single aperture that is much smaller than a wavelength, because there is negligible destructive interference.

[jtbell]

As the aperture gets smaller, the central maximum in the diffraction pattern spreads out (becomes wider). When the aperture becomes small enough, the central maximum completely fills the space beyond the aperture. The aperture effectively becomes a point source of light, radiating uniformly in all directions beyond the screen.