I'm reading this tutorial on instructables for implementing the classical double-slit experiment. I've also read this thread for information which contains a very nice answer from @BruceW but still not resolves my confusion about the "sizes". First of all I'd like to make a distinction between "longitudinal slit width" and "transverse slit width(s)". I believe the @BruceW answer for the referred thread is convincing for that "longitudinal slit width should be comparable to wavelength of light". Hence I'm asking mostly about "transverse slit width(s)". In the tutorial on instructables, it's said that one can make double-slit by Which seems ambiguous in "utility knife". I have a box-cutter utility knife at hand but its blade tip seems nothing comparable to 400nm ~ 700nm as I can clearly see. On the transverse plane of double-slit experiment, do I have to cut both transverse slit widths to nearly wavelength of light? If yes how did Young make it back at his time? Or else how long should they be cut? Is this relevant to light-wavelength, beam-transverse-width(s) or polarization?