Question

How does the single-slit diffraction pattern look in comparison to the double-slit interference pattern? Is it possible to have a double-slit pattern without the single-slit pattern overlaid?

Answer 1

One way of understanding this which has always had intuitive appeal to me is the so-called Huygens Principle which basically states that every point on a wavefront can be considered a point source for a new spherical wave, and that the evolution of the wavefront can be determined by superposing all of these spherical waves at later times. The Wikipedia article that I linked to has some really nice pictures of this.

Diffraction effects can then be explained using this principle. Imagine, for example, that you shine light through an extremely small slit, say a slit about the size of the wavelength itself, then when plane waves pass through this slit, the part of the wave that goes through the slit acts as a point source and generates a spherical wave, so the light diffracts.

If the slit is larger, however, then the part of the wavefronts that pass through the slit act as multiple little points sources for spherical waves, and these spherical waves interfere with each other to give an interference pattern. In this way, the diffraction pattern is very much like multiple slit interference, except instead of multiple slits, the wave front itself splits into a bunch of adjacent point sources that interfere with each other.