= Solution
For a spherical grain of diameter $D$, density $\rho_d$, and <radiation-pressure efficiency> $Q_{\rm pr}$, comparison of the stellar momentum flux with <Newton's law of universal gravitation> gives the <radiation-pressure coefficient>
$$
\boxed{\beta(D)=\frac{3L_*Q_{\rm pr}(D)}{8\pi cGM_*\rho_dD}}.
$$
For $D$ much larger than the characteristic stellar wavelength, $Q_{\rm pr}\simeq1$, so $\beta\propto D^{-1}$. When $D$ becomes comparable to the optical wavelength, diffraction and the grain's composition make $Q_{\rm pr}$ size dependent and $\beta$ reaches a broad maximum. Deep in the <Rayleigh scattering> regime, absorption can give $Q_{\rm pr}\propto D$ and hence nearly constant $\beta$, while scattering alone gives $Q_{\rm pr}\propto D^4$ and hence $\beta\propto D^3$. A realistic curve therefore rises roughly as $D^{-1}$ toward micron sizes, turns over near the stellar spectral peak, and falls or flattens for still smaller grains.
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