3.9 understand that waves can be diffracted through gaps, and that the extent of diffraction depends on the wavelength and the physical dimension of the gap.

The extent of diffraction depends on 2 things:

1. Wavelength
2. Dimensions of gap

If the wavelength is greater, the more the wave will diffract.

As you can see here the width of the gap varies diffraction. The smaller the gap the more the diffraction.