Diffraction angle goes as wavelength over aperture, so halving the opening doubles the spread — squeezing a beam tighter makes it fan out wider, not narrower.
It sets a hard floor on how sharp any optical instrument can be. A pinhole camera cannot be improved indefinitely by shrinking the pinhole, and a telescope resolves finer detail only by getting physically bigger — no amount of magnification substitutes for aperture.
The relationship is for the far field, where the screen is many aperture-widths away; close to the opening the near-field pattern looks quite different. It also assumes the aperture is much larger than the wavelength — comparable sizes need a full electromagnetic treatment, not this approximation.
Estimate with the rule, then check it against the calculator that models it properly.
Open Refraction & Diffraction Calculator →Diffraction angle goes as wavelength over aperture, so halving the opening doubles the spread — squeezing a beam tighter makes it fan out wider, not narrower. It sets a hard floor on how sharp any optical instrument can be.