That is where the numbers come from, the 'why' is more interesting. To block a transverse  EM wave we need to present it with a short circuit plane - we need to connect together regions of space that would otherwise support a voltage gradient. This could be a continuous sheet of metal of conductor, but it could be reduced to just joining a few nodes to connecting regions in space that would be at different voltages if the wave was present, and if we do this will prevent the field building.
This use of mesh instead of sheets is like approximating something curved by a series of flat pieces. How badly connected those flat pieces can be before the 'short circuit' is not good enough to suppress wave propagation  can be seen as an example of sampling theorem. The tenth of a free space wavelength is a bit of a rough rule of thumb, it could be 1/8 or 1/20, and the degree of attenuation will change - getting better with more metal and less holes, but much by the time you have an aperture of half a wave, you can propagate a full strength wave through it - Nyquist.

Or if you prefer, you can sample a sine wave at 10 dots per cycle it looks like a wave if you sample it at only 2 dots per cycle it looks like DC.

Mike

A nanosecond is about 1ft in free space.

That is where the numbers come from, the 'why' is more interesting. To block a transverse  EM wave we need to present it with a short circuit plane - we need to connect together regions of space that would otherwise support a voltage gradient. This could be a continuous sheet of metal of conductor, but it could be reduced to just joining a few nodes to connecting regions in space that would be at different voltages if the wave was present, and if we do this will prevent the field building.
This use of mesh instead of sheets is like approximating something curved by a series of flat pieces. How badly connected those flat pieces can be before the 'short circuit' is not good enough to suppress wave propagation  can be seen as an example of sampling theorem. The tenth of a free space wavelength is a bit of a rough rule of thumb, it could be 1/8 or 1/20, and the degree of attenuation will change - getting better with more metal and less holes, but much by the time you have an aperture of half a wave, you can propagate a full strength wave through it - Nyquist.

Or if you prefer, you can sample a sine wave at 10 dots per cycle it looks like a wave if you sample it at only 2 dots per cycle it looks like DC.

Mike

A nanosecond is about 1ft in free space.