waves - Why do we demand that $\lambda \ll \text{size of slit

I'd appreciate it if anyone could explain to me why do we demand that the wavelength will be much smaller than the distances between the slits in order to see wave diffraction when performing Young's

Solved 1. For a single slit, how must the size d of the slit

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Why is the number of wavelets in a slit = slit length / lambda

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