Figure: Illustration and explication of diffraction dependence on wavelength-aperture ratio

Caption: The image illustrates how the diffraction pattern for 580 nm light (i.e., yellow light) of a single slit aperture depends on the ratio of wavelength to slit width. The original caption for this image is not complete. Yours truly thinks it is simulated image, not a set of photographs of real diffraction patterns.

Further explication:

1. Note, in diffraction patterns regions of constructive interference are often called bright fringes whether they look like fringes or not. Similarly, regions of destructive interference are often called dark fringes whether they look like fringes or not. The diffraction patterns for slits when projected on a screen actually do look like fringes. For a single slit, the central bright fringe is the dominant fringe.

2. The ratios for the shown the diffraction patterns going downward are described as follows:
   1. 1 over 1000. In this case, diffraction pattern is basically just a image of the slit. The central bright fringe completely dominates and the side constructive interference bright fringes and destructive interference dark fringes are squeezed very close to the central bright fringe or they are very faint, and so are are invisible in the image. This case is close to the limit of ideal geometrical optics where light is just treated as light rays.
   2. 1 over 30. The slit width is decreased and now the central bright fringe is wider than the slit and side fringes are visible
   3. 1 over 6. As the slit width is decreases the pattern spreads out.
   4. 1 over 3. More spreading out.
   5. 1 over 1. Now the central bright fringe occupies most of the image with just one pair of dark fringes on the sides of the central bright fringe However, the full pattern is likely not known since there is not enough description in the original caption.

3. The overall single slit diffraction scales in size with what yours truly calls the diffraction ratio R introduced in Optics file: Figure: How the diffraction ratio describes the transition from physical optics to geometrical optics.

   In fact, the diffraction ratio for a single slit diffraction is rather exactly the angular position relative to the center of the pattern of the minimum intensity of the first dark fringe.

   For example for R = 1 over 1000 radians = approximately 1 over 14 in degrees is very small angle to casual observation. For such a small angle all the fringes that are not too faint to see other than the bright central fringe are very thin and squeezed very close to the bright central fringe, and so invisible to casual observation which, in fact, is what is seen in the image.

   On the other hand, for R = 1 over 60 radians = approximately 1 degree and the corresponding dark fringe is clearly seen in the image.

   For R = 1, all you see in the image is the bright central fringe and the first dark frinees.

   If R → ∞, then the central bright fringe would become a hemispherical wave pattern as from a point light source on an opaque wall.???

4. The detailed structure about a diffraction pattern can only be determined by calculation. However, an aperture or obstacle of high symmetry imposes that symmetry on the diffraction pattern. Another perspective is that given the a highy symmetry aperture or obstacle, there is nothing to break the symmetry in the process of diffraction pattern formation.

   For a single slit aperture, the pattern is as in the image: long fringes symmetrically arranged around a broad central fringe. For a square aperture, the pattern will have square symmetry For a circular aperture, the pattern will have a circular symmetry.

5. Slit-like diffraction patterns that are somewhat explicated above are illustrated in the very good silent video:
   Young's Double Slit Experiment | 1:09: Well, Jack Maxwell (the great, great, great grandson of Jim Maxwell) never uses slits, but it's still pretty nifty what you can do with laser pointer and some pencil leads (i.e., graphite cores). See Wikipedia: Young's interference experiment for the real Young's interference experiment. Actually rather than using slits, Jack M is relying on Babinet's principle to substitute for slits. Note Babinet's principle: "the diffraction pattern from an opaque body is identical to that from a hole of the same size and shape except for the overall forward beam intensity" (see Wikipedia: Babinet's principle). Very good for the classroom.