= Solution
{c}
<Gravitational waves> also exhibit <diffraction>: the <geometrical optics> ray description fails when a propagation structure or lens delay is not large compared with the wave's <wavelength> or period. For a compact lens of <mass> $M_L$, a useful <dimensionless> parameter is $w=4GM_L\omega_{\rm gw}/c^3$; $w$ of order one signals potentially appreciable wave effects, with cosmological lens redshift supplying an additional factor $1+z_L$ when needed. Astrophysical <gravitational lensing> can therefore produce <diffraction> and frequency-dependent amplification. Ordinary objects are poor opaque screens for <gravitational waves>, so everyday slit-and-screen demonstrations are implausible despite the large <wavelengths>, which are about $3000$ km at $100$ Hz.
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