I've been studying waves and diffraction has come up, I'd just like to

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slingboi
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I've been studying waves and diffraction has come up, I'd just like to know, why is it that diffraction is most significant when the size of a gap a wave passes through is similar to the wave length?
 

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As you probably know, when waves pass through a gap they 'spread out' and produce a diffraction pattern consisting of maxima and minima.
If the gap is one wavelength wide (a slit) then it can be shown that the FIRST MINIMUM is when θ= 90. This means that the slit behaves essentially as a 'point source' with waves spreading out in all directions.
If the slit width is 2λ then the first minimum is at 30 and the second minimum is at 90 so a diffraction pattern is produced with max and min.
Anything less than one wavelength wide produces a 'point source' like pattern so a slit of width 1λ is significant in diffraction regarding the details of the pattern produced.
If the slit is 10λ wide then the first min is at 5.7 degrees and you could say that the waves are more or less passing straight through