  The far-infrared (FIR) opacity of dust in dark clouds within the Ophiuchus
molecular cloud is investigated through multi-wavelength infrared observations
from UKIDSS, Spitzer and Herschel. Employing the infrared color excess
technique with both near-infrared (NIR) and mid-infrared (MIR) photometric
data, a high-resolution extinction map in the $K$ band ($A_K$) is constructed
for three dark clouds: L1689, L1709, and L1712. The derived extinction map has
a resolution of $1'$ and reaches a depth of $A_K\sim3$ mag. The FIR optical
depths $\tau_{250}$ at a reference wavelength of $250\,\rm \mu m$ are obtained
by fitting the Herschel PACS and SPIRE continuum data at 100, 160, 250, 350 and
500 $\rm \mu m$ using a modified blackbody model. The average dust opacity per
unit gas mass at $250\rm \mu m$, $r\kappa_{250}$ is determined through a
pixel-by-pixel correlation of $\tau_{250}$ with $A_K$, yielding a value of
approximately $0.09\,\rm cm^2\,g^{-1}$, which is about 2-3 times higher than
the typical value in the diffuse interstellar medium (ISM). Additionally, an
independent analysis across 16 sub-regions within the Ophiuchus cloud indicates
spatial variations in dust opacity, with values ranging from 0.07-0.12$\,\rm
cm^2\,g^{-1}$. Although the observed trend of increasing dust opacity with
higher extinction implies grain growth, our findings indicate that rapid grain
growth clearly not yet occurred in the dark clouds studied in this work.
