Caption: "A computer simulation of the diffraction pattern formed by a laser beam of wavelength 663 nm = 0.663 μm (1 m = 10**(-6) μm = 10**(-9) nm) incident on a square aperture of 20 X 20 μm visible on a screen placed 1 meter from the aperture." (Slightly edited.)

Features:

1. The displayed diffraction pattern measures 30 cm X 30 cm.

2. Note the relatively small aperture is what gives rise to the big diffraction pattern.

   But it is NOT a very small aperture. The ratio of wavelength to aperture size scale is 0.663/20 = 0.03315. So ∼1.5 orders of magnitude less than 1 which is smallish, but still NOT tiny. So one gets a noticeable diffraction pattern.

3. If the aperture were progressively scaled up, the central bright fringe would expand into a big square beam of light that would be practically an image of the aperture. The interference fringes would would narrow down into near invisibility becoming very faint, except very near the bright central fringe.

   With reflections and other sources of light, the interference fringes would be washed away.

4. The particular shape of the diffraction pattern is a result of the square aperture. Different apertures would give different shapes. A circular aperture would give a diffraction pattern with circular symmetry (which is called an Airy diffraction pattern) as it must since there is nothing in the aperture to break circular symmetry.

5. Further elucidation is provided by the Young's Double Slit Experiment | 1:09 video below.

   EOF