Say you have two openings close to each other in a wall. Why does the length of the openings have to be equal to or less than the wavelength of the waves in order to create an interference pattern?
The lengths of the openings do not need to be equal or less than the wavelength for there to be a fringe pattern. For instance in a double slit experiment with 500nm wavelength light and slits that are 100,000nm wide, separated center to center by 200,000nm you will get (interference) or a fringe pattern with 2,500,000nm spacing's. If you change the separation of the slits the spacing of the fringe pattern will change. Interestingly if you change the width of the slits but not the separation the double slit pattern will stay the same. There is also the single slit pattern to consider like in the experiment I described above you will also find a pattern every 5,000,000nm. You can calculate the spacing of any fringe pattern for any multiple slit experiment, even single edge fringe patterns which are different and do not have equal spacing's but consistently get smaller.